TVTropes tl;dr
Because I ended up on TVTropes and that site is the ruin of many a life, unfortunately, and mostly because I can and was bored of work.
Not in any particular order except 'whatever occurs to me atm'.
The Purity Sue. Unless you squint, if you do it's actually subverted to hell and back. But seriously, the only character who met Seth and didn't comment on his pure purity and beautiful inner beauty was Art, and Art was the sort of person who preferred cute little boys wheedling him for a party to the most beautiful girl in the world wheedling him for a kiss. (If that sounds wrong, that's probably because it is.)
Incorruptible Pure Pureness. See above. Subverted, see below.
Fridge Horror. What happens when you squint. It's actually pretty disturbing when you realize that what he means and what everyone thinks he means are essentially different things. Seth is the least humanlike of the three mermaid siblings and the only one who said that mermaids have no soul. Combined with how he actually thinks and this is horror material.
Stepford Smiler. When you take into account everything, he's actually this even if the author tends to pretend that he's something else. He spent most of canon as a Type B trying to turn into a Type A. And it gets worse from there.
Too Good For This Sinful Earth. Played straight, then inverted. Seth literally died because the Earth was too polluted and he cared too much about his siblings to take care of his own sickness. He even coughed up blood, man, a Sorry That I'm Dying moment and everything. Then he subsequently get revived to much better health, with the payment for his life being that of every single life on Earth. Not that he knows about this, mind.
Kuudere. Type 2. Almost all of Seth's expression icons come from one scene in the whole series. Most of the time, he either smiles his Stepford Smile in that mild-mannered way or looks prettily expressionless. Most of the time, he's very, very kuu, and slightly veering into tsun if he's annoyed. Most people who read the manga remembers him as a fidgety blushy thing, however, since he kind of spent the last four volumes in adorable dere mode. And those last four volumes happened to be very, very memorable.
Yamato Nadeshiko. Once you get past the kuu, he's generally this. His dere is hard to get, okay.
Actual Pacifist. He would rather starve to death than eating fish that his brother killed, how's that for pacifism? He doesn't have any problems with causing you permanent trauma by throwing extremely lifelike illusions of giant constrictor snakes eating your face, however.
Evil Cannot Comprehend Good. Inverted. In this case, good cannot comprehend evil. This is played for both lulzly and disastrous results. And actually, it's less 'good cannot comprehend evil' than Seth lacking a real moral compass. He doesn't differentiate between a good act that hurts someone and an evil act that does the same thing, in a way. He's good, but it's not because he's good, if you catch my drift.
Ambition is Evil. Subverted. He thinks it's evil and acts on that thought. Actual evil and sadness for everybody involved (audience included), however, is derived from his conclusion that ambition is evil.
Apologises A Lot. While he hasn't reached the point of apologizing for being born yet, he does apologize a lot. One of his trademark phrases----which kind of carried over into the author's following series, either as an in-joke or a personal moe----is 'suimasen'. Which is kind of like a formal apology that doesn't quite make it all the way to formal.
I Just Want To Be Beautiful. One of his main angsts is either this or actual gender dysphoria, which wouldn't surprise me. Ironically, everybody else in the family wishes they could be as beautiful as Seth. He himself isn't unattractive, either...it's just, you know, he's not a girl.
Did You Think I Can't Feel. Played straight for his romantic relationship. Subverted for his everything else. He feels the resentment, yes, he just never voices them out loud.
Lawful Good. A little bit subverted in that his central conflict isn't the axis between Law/Good or Good/Evil, it's actually Good/Good. And a potent reminder that good can be nice and actually fucking scary at the same time (but only when you think about it).
Dojikko. It doesn't always make it through the translation, but canon stated several times that he is.
Has Two Mommies. Played somewhat straight. His actual mother is dead (see below), he's raised by his two aunts, ovaryless twins of his mother (much in the same way as Teruto is to him and Benjamin).
Sorry Billy But You Just Don't Have Legs. Or a functioning ovary, for most of the story. Due to the way the biology is set up and the way the story works, however, the audience doesn't quite know what to think, either, when The Lack of Legs turns out not to matter to mermaids after all.
Oblivious Guilt Slinging. All the three mermaid siblings do this to each other. Seth's case was particularly bad, though, as there are hints that he sometimes knows perfectly well that it would make the other party feel guilty. He just doesn't consciously recognize that what he says is guilt-tripping. You know. Like every guilt-tripping ever.
Innocent Aliens. While the degree of their innocence can be debated, all the mermaids invariably think of themselves as aliens, and the ocean of stars is their true home. Seth's half human, however, so his opinion on this is kind of strange from both the human and alien point of view.
The Mind Is a Plaything of the Body. Seth is Really Six Hundred Years Old. He spent a large part of that as a child, however, unlike most of his racial compatriots, and while he was a child he really was a child. A quiet and kind and thoughtful child who may think about Deep Things more often than usual, but still a child who didn't comprehend death (or at least viewed it very differently than how he's currently seeing it) and preferred painting with crayons to visiting art museums. He only became what you currently see six months prior, actually, and only started to act as such around the same time. It's kind of hard to say how mature MiniSeth was, since we didn't see much of him---we do know that Seth and Teruto didn't expect MiniBenjamin to act like an actual 12-year-old, but we know that they weren't really adults, either---but his state of mind is definitely affected by the form he's in. (It's also possible that MiniSeth was a Stepford Smiler Type B for 600 years---there are certainly hints of that----and he only tried to turn into a Type A for the last six months of his life. If this is true, the whole thing becomes even worse than it already is. He was a little kid.) If this is normal for his species, it may explain somewhat why the gender issue is a Big Deal. And why---although he is essentially the most 'female' out of the entire cast, there are some parts of Seth that are undeniably boyish in nature.
Took A Level In Badass. Unlikeliest thing ever, I know! But it happens. As this is after his canon point, however, he might never get there as far as RP is concerned. I like playing wimpy wimps.
While Rome Burns. Not really his fault as he was presumably kept in the dark while it actually happened, but during the 200 years when all the mermaids returning to Earth were systematically slaughtered---a long, thorough genocidal process that repeats itself every time they returned, unknowing of the dangers that awaited them there----Seth was on the moon, growing up happily and presumably enjoying tales of cute pink fluffies. It boggles the mind when you consider how much Sara and Miralda stressed the fact that the children (Seth and Benjamin, basically) have to become beautiful women and purify their tainted-with-human bloodline from that accursed prince so their sins could be atoned for-----while not lifting a finger to warn the mermaids and prevent the near-extinction of their race. This is made better by the fact that Seth's parents were the ones who initiated the genocide, and his attitude to it (even currently) is.........extremely detached, to say the least.
Green Eyed Monster. All the mermaid siblings are this to each other, but Seth's case was made more noticeable than the rest because it's one of the central cruxes of his dilemma. If you ask me, Teruto's case was in fact stronger and more complicated, but fans tend to ignore that in favor of the Gay Interpretation (which I think is actually part of the problem instead of a separate issue caused by fan filtering. Teruto's actions and reactions are all sort of wrong if the official explanation is all it is). Hilariously enough : Seth's eyes are canonically blue, but the author has a tendency to color them green.
Always Second Best. The main reason for him being a green-eyed monster. Seth doesn't think much of being the nicest person ever, no.
Backup Twin He is meant to be this, literally.
Second Love. He ends up being this to Shonach, and you'll root for it anyway despite seeing it coming like several miles away. The explanation for the 'second love is better than the first' clause here is that Shonach's first love was based entirely on looks and he never saw the real Benjamin behind the pretty girl, but in Seth's case it's more or less the reverse, hilariously enough. He was pretty attached to Seth from the get-go, probably because Seth's personality is that of Shonach's dream girl, but he took his time noticing the girl hidden under that boy face. To add insult to the injury, the main reason why Shonach fell in love with Benjamin was due to subliminal conditioning from dreaming about 'Seira's daughter' for six centuries, and Benjamin was that image to a tee, so he was in over his head that she's his destined love. But. As Seth is actually also 'Seira's daughter', he'd turn into exactly the same woman Shonach saw if Benjamin is out of the picture, and his personality fits her better than B's did. So who did Shonach actually see? Was it just an epic case of mistaken identity where he's supposed to have fallen in love with Seth all along and was just tested by fate? Was it the witch's curse? To add even more insult to the injury, you could make a strong argument that Shonach actually preferred the Boy!Seth that he fell in love with to the Girl!Seth that he thought he loved, although it didn't matter by then anyway. Destined/manipulated or not, they somehow managed to fall in love out of their own accord. There's a bit of subversion here, of course, in that the second love doesn't negate the first's importance, much to Seth's chagrin.
True Beauty is On the Inside. Played straight to hilarious degrees.
Our Love is Different. Subverted. He notices Shonach's Casanova tendencies and snarks about it all the time. Shonach's niceness to him, however, is really genuine person-to-person niceness, as for most of the story he's not even considered a potential romance target....and Seth just kind of never really notices it. He even denies being treated differently from the rest of the world by Shonach when it's pointed out to him.
Stay With Me Until I Die. Subverted in that he actually doesn't say it out loud due to knowing that the one he wants to say it to just doesn't see him that way, but he certainly hinted at it. It's just his greatest unspoken wish, one that remains as his greatest impetus even in this RP. Subverted again in that it's actually pretty mutual, only Shonach also has this thing with Dying Alone and they both believe that to demand something more would be entirely wrong (for different reasons), neither of them said a thing and were prepared to never see each other again as Seth left for Russia. Even when Shonach came over (to rescue Seth, not that he needed rescuing) and they both spent their last days together, neither of them realize that this trope is each other's wish. Ow. So that makes for an Esoteric Happy Ending when (spoilers) Seth's 'good' ending (the canon has two endings, the reader gets to pick) was a Someone to Remember Him By ending instead, with possibly one of the most heart-rendingly subverted Dying Declaration of Love and Never Got to Say Goodbye that I've ever read. It has all the suggestions of a good end as the genre allows, complete with The Power of Love-induced miracles (in that he's allowed to live without wrecking the Earth) and part of the audience treats his getting a kid as a good thing. But when you think of how many times he actually mentioned, thought, and in all ways implied that he only wanted to lay eggs in context of his love for Shonach and what he really wants is to spend every last minute of his life with his beloved---coupled with the fact that he never contacted Benjamin again, and the fact that breeding females cannot ever commit suicide---you wonder if he would've chosen this ending, if he could. I personally see him spending that five years raising Teruto Junior with a billion tons of Survivor's Guilt, and am glad that he didn't have to endure it for longer than that. You know, for a person with this as a True Wish (tm), it kind of sucks that everyone he loves find it necessary to die so he could live a little bit longer. Teruto, Shonach, I'm looking at you two.
Be Careful What You Wish For. Played pretty damned straight, with the clause that what he wished for wasn't his true wish, by the end. Interestingly, (spoilers) Shonach was the one who ended up with Seth's true wish, to die in the arms of the person he loves, and he seems pretty happy about it. Seth ended up with Shonach's true wish, too, to lay eggs with the person he loves. It's unknown whether Seth is happy about the deal, but at least Teruto Jr. (this is because Seth named the child 'Teruto' after his favourite brother, not that Teruto/Gill knocked him up or anything) seems like a very loved child, so at least it's a World Half Full.
Hermaphrodite. Played straight to the logical conclusion, very seriously. Has been known to break the brain of readers who obligated the questions posited by canon and think really hard about it. Seriously, the sexuality of mermaids is really really weird when you think hard about it.
Our Mermaids Are Different, now that we're on the subject. When you think of mermaids, you don't think of sixteen-year-old boys in white tuxedos who turn into salmon-like Space Whales (but only, you know, salmon-sized) who swim faster than light, do you? By the way, that suit doesn't need changing. Tuxedos are obviously the new mermaid's tail. (It makes you wonder, considering that Seth was born in the 14th century : where did the tux come from? And also : if they have legs anyway, what the hell did Seira sacrificed her voice for? Is it related to the above point? Does she need extra gear to mate with a human? Gah!)
Birth Death Juxtaposition. The entire mermaid race lives on this, but his case is pretty special.
We Are As Mayflies. Despite the fact that the typical mermaid lifespan is 200-800 years, and the fact that Seth has lived longer than some countries, you still get the impression that his time (and all mermaids) is short-lived and fleeting. This is probably because they spent forever in 'naiad' stage and turned into adults only to mate and die. Probably is not helped by how, though the other mermaids got to live out teenagerhood and all the cool things on other planets and see the wide universe, Seth was stuck in a cave on the moon until recently. He's never even seen an ocean! He cries at seeing one, he's never had friends, never lived life to the fullest. It adds to the effect, really.
Meaningful Name. 'Seth' was, according to Google, the name of Adam and Eve's third son, 'appointed to replace Abel, after he is killed by Cain'. While this seems like an inversion at first, this is played pretty straight in several ways. He is Benjamin's genetic replacement in case she dies, after all, and Teruto (Cain) only spent 13 volumes trying to kill Benjamin off. Also : in Japanese, he's written 「セツ」 or 'Setsu', which is the on-yomi reading for 'snow'. Guess what substance he is compared to the most in the story. And also, there's a saying in whatever remains of J-Fandom that 'Setsu stands for Setsunai' (「セツ」は「切ない」の「せつ」), which, if you know the the meaning thereof, is probably the most accurate one-phrase description of Seth ever.
All Love Is Unrequited. Is one of the points of the manga. I mean. Er. EVERYBODY seriously knows what everybody is feeling toward each other except maybe Art because he can't wrap his head around the concept that Hot Girl = Cute Shotaro Boy. And Rita because she just doesn't know, though she sort of guessed that this 'Seth' person was someone very special to Gill-sama. But it goes like this. Rita loves Gill/Teruto, who loves Seth, who loves Shonach, who loves Benjamin, who loves Art, who loves Jimmy (not in that way, but it totally counts). Due to one reason or the other, none of this goes anywhere until near the end.
A Tragedy of Impulsiveness. Subverted and played straight. The root causes of all tragedies is Seth thinking too much. The events that lead to them is because of Seth thinking too little. The kid obviously needs a middle ground.
Dreaming of Things To Come. Not as big a part of his plot as Benjamin, due to him giving less of a shit about humans than she did, and due to the relevant decision not being in his hands. It is still a part of his plot, however, and he agonized over it quite a fair deal.
Power Trio. Teruto is the cold logical one, Benjamin is the nice humanistic one and Seth is the rational intuitive one. Back when the three were actually living together, Seth actually played balancer and peacemaker between his two siblings, and there it's played straight. As Benjamin gets amnesia and Teruto disappears into his World Destroyer Plan, however, this trope is inverted to hell and back and balancing the opposites become the last thing that mattered in his plot. Being thrown out of his original role basically made Seth feel lost and useless, however, and left him emotionally wide open for all sorts of nasty things. Like falling in love. Which also is a clause in world-ending curse, mind.
The Hecate Sisters. Teruto is the sharp-witted crone (although he talks like a guy) and actually has dealings with a real witch, Benjamin is the blonde and naive ditz, and Seth is the incredibly maternal mother.
Cain and Abel It started as sibling rivalry (and it's sibling rivalry like whoa, too), but there comes a point where everybody wants to kill Benjamin.
Middle Child Syndrome. Subverted. When they were still family, it's Seth who was treated more like the youngest child by Teruto, while Benjamin was treated like the youngest child by their parents and both felt pretty alienated from each other. The real reason for his Middle Child Syndrome is the fact that he has always been sickly and know that he'll never succeed----unlike the Oldest Child, who usually at least succeeds at being himself, and the Youngest Child, who succeeds because they do....
Youngest Child Wins ....Guess which child Seth is? No, he's not youngest.
Big Brother Worship Seth adores Teruto, however. I don't know if this is better or worse by the fact that he knows full well what Teruto's faults are, if his dying requests are any judge. Oh, he's envious like whoa, but there's no question that for 600 years, Teruto was his everything. He even sort of half-stated that he might not have an eye for Shonach if Teruto is still around. It's probably because in that case he would show his emotional vulnerability to nobody else except his twin, but still.
Living Emotional Crutch to Teruto, in return.
Different as Night and Day for Seth and Teruto, although they seemed to be Creepy Twins at first. They, uh, actually have Twin Telepathy and Wonder Twin Powers, too, and the Twin Banter is the only times you'd ever find Seth talking in a really masculine way (not as much as Teruto's use of 'ore', though) and actually relax. And they both end up with different versions of Angsty Surviving Twin. There's also a healthy dose of Twincest, although how much of this is actual and how much of is just mermaids being weird is kind of up in the air. It is shown, however, that they're not actually different as night and day after all. It's just that each has placed the other as their ideal, and then ends up feeling super inadequate and then perpetuates the difference even more. Why do I say twins even though they're triplets? Because, really, as far as Seth and Teruto are concerned, they're pretty much twins and Benjamin is the younger sibling. B really was the odd man out, which makes his situation Fridge Tragedy when you think about it; is there any surprise that B jumped at familial warm the moment he runs into one?
Heroic BSOD. This happened when Teruto disappeared, even if it's handled somewhat subtly. He got better after meeting Shonach and getting reminded that the world still exists. Predictably, he falls in love and it rolls downhill from there. (This is remarkable, as Shonach is actually kind of a cold superficial jerk, albeit a very devoted and romantic one, and the lines that Seth originally interpreted as kindness was never meant that way-----and he realized they were never meant that way. It actually ties into something else in his character, but that one is rare enough that it's not a trope.)
Apocalypse Maiden. All three of the siblings, in different ways. In Benjamin's case, it's because Destiny Says So. In Seth's case, it's because of Black Magic. In Teruto's case, it's because he's a Woobie Destroyer of Worlds. Nevermind that Apocalypse, in this case, is more or less a Space Whale Aesop. I mean. This is a story about three actual Space Whales, man.
Beta Couple, Breakout Character. While it's obvious that Shonach/Seth was intended to happen since the story direction changes in vol.2, the fan popularity and author investment (screentime) in the pair eventually eclipsed the main couple itself. By the ending, they're basically treated as the main 'romance', and way back when I read it there was a saying going round that 'Moon Child is a manga you read for the secondary pairing'. I'm not even kidding. I appreciate Art a whole lot better now, but it's seriously like this : 90% of the fandom ship Shonach/Seth, 9% ship Teruto/Seth and 1% ship Art/Benjamin. There were more Art/Benjamin shippers when it was running, if author commentary is to be believed, but nobody really remembered them 20 years later.
Jumping the Gender Barrier. When you've been rooting for it for pretty much the whole story but you can't decide what to feel when it finally happens, I tip my hat off to the author and ask sweetly if I can burn her house.
Yaoi Guys. Consequence of the above, and visually. I confess that I seriously can't see Seth as a male character, despite the fact that he is vastly more easily understood as a boy and I do treat/write him as one, and so this doesn't really apply to how I interprete him. His relationship, as far as I'm concerned, is het, and I treat him as a bit of a transgendered person. See this picture? If you haven't read the series, that's a bona fide BL picture. If you've finished reading, then they're the most star-crossed hero and heroine in the history of ever. Seriously. You will, by the end, see Seth as a heroine instead of as a Yaoi Guy. But it's there, as a shoujo manga trope. And there is still debate about this in Japan---whether to consider Shonach/Seth BL or het---23 years after the publication. It's not a straight case of genderbent because Seth isn't really a girl either, and not just in looks. But the trope, it's there.
Your Days Are Numbered. Ultimately, the reason behind everything he does.
Not in any particular order except 'whatever occurs to me atm'.
The Purity Sue. Unless you squint, if you do it's actually subverted to hell and back. But seriously, the only character who met Seth and didn't comment on his pure purity and beautiful inner beauty was Art, and Art was the sort of person who preferred cute little boys wheedling him for a party to the most beautiful girl in the world wheedling him for a kiss. (If that sounds wrong, that's probably because it is.)
Incorruptible Pure Pureness. See above. Subverted, see below.
Fridge Horror. What happens when you squint. It's actually pretty disturbing when you realize that what he means and what everyone thinks he means are essentially different things. Seth is the least humanlike of the three mermaid siblings and the only one who said that mermaids have no soul. Combined with how he actually thinks and this is horror material.
Stepford Smiler. When you take into account everything, he's actually this even if the author tends to pretend that he's something else. He spent most of canon as a Type B trying to turn into a Type A. And it gets worse from there.
Too Good For This Sinful Earth. Played straight, then inverted. Seth literally died because the Earth was too polluted and he cared too much about his siblings to take care of his own sickness. He even coughed up blood, man, a Sorry That I'm Dying moment and everything. Then he subsequently get revived to much better health, with the payment for his life being that of every single life on Earth. Not that he knows about this, mind.
Kuudere. Type 2. Almost all of Seth's expression icons come from one scene in the whole series. Most of the time, he either smiles his Stepford Smile in that mild-mannered way or looks prettily expressionless. Most of the time, he's very, very kuu, and slightly veering into tsun if he's annoyed. Most people who read the manga remembers him as a fidgety blushy thing, however, since he kind of spent the last four volumes in adorable dere mode. And those last four volumes happened to be very, very memorable.
Yamato Nadeshiko. Once you get past the kuu, he's generally this. His dere is hard to get, okay.
Actual Pacifist. He would rather starve to death than eating fish that his brother killed, how's that for pacifism? He doesn't have any problems with causing you permanent trauma by throwing extremely lifelike illusions of giant constrictor snakes eating your face, however.
Evil Cannot Comprehend Good. Inverted. In this case, good cannot comprehend evil. This is played for both lulzly and disastrous results. And actually, it's less 'good cannot comprehend evil' than Seth lacking a real moral compass. He doesn't differentiate between a good act that hurts someone and an evil act that does the same thing, in a way. He's good, but it's not because he's good, if you catch my drift.
Ambition is Evil. Subverted. He thinks it's evil and acts on that thought. Actual evil and sadness for everybody involved (audience included), however, is derived from his conclusion that ambition is evil.
Apologises A Lot. While he hasn't reached the point of apologizing for being born yet, he does apologize a lot. One of his trademark phrases----which kind of carried over into the author's following series, either as an in-joke or a personal moe----is 'suimasen'. Which is kind of like a formal apology that doesn't quite make it all the way to formal.
I Just Want To Be Beautiful. One of his main angsts is either this or actual gender dysphoria, which wouldn't surprise me. Ironically, everybody else in the family wishes they could be as beautiful as Seth. He himself isn't unattractive, either...it's just, you know, he's not a girl.
Did You Think I Can't Feel. Played straight for his romantic relationship. Subverted for his everything else. He feels the resentment, yes, he just never voices them out loud.
Lawful Good. A little bit subverted in that his central conflict isn't the axis between Law/Good or Good/Evil, it's actually Good/Good. And a potent reminder that good can be nice and actually fucking scary at the same time (but only when you think about it).
Dojikko. It doesn't always make it through the translation, but canon stated several times that he is.
Has Two Mommies. Played somewhat straight. His actual mother is dead (see below), he's raised by his two aunts, ovaryless twins of his mother (much in the same way as Teruto is to him and Benjamin).
Sorry Billy But You Just Don't Have Legs. Or a functioning ovary, for most of the story. Due to the way the biology is set up and the way the story works, however, the audience doesn't quite know what to think, either, when The Lack of Legs turns out not to matter to mermaids after all.
Oblivious Guilt Slinging. All the three mermaid siblings do this to each other. Seth's case was particularly bad, though, as there are hints that he sometimes knows perfectly well that it would make the other party feel guilty. He just doesn't consciously recognize that what he says is guilt-tripping. You know. Like every guilt-tripping ever.
Innocent Aliens. While the degree of their innocence can be debated, all the mermaids invariably think of themselves as aliens, and the ocean of stars is their true home. Seth's half human, however, so his opinion on this is kind of strange from both the human and alien point of view.
The Mind Is a Plaything of the Body. Seth is Really Six Hundred Years Old. He spent a large part of that as a child, however, unlike most of his racial compatriots, and while he was a child he really was a child. A quiet and kind and thoughtful child who may think about Deep Things more often than usual, but still a child who didn't comprehend death (or at least viewed it very differently than how he's currently seeing it) and preferred painting with crayons to visiting art museums. He only became what you currently see six months prior, actually, and only started to act as such around the same time. It's kind of hard to say how mature MiniSeth was, since we didn't see much of him---we do know that Seth and Teruto didn't expect MiniBenjamin to act like an actual 12-year-old, but we know that they weren't really adults, either---but his state of mind is definitely affected by the form he's in. (It's also possible that MiniSeth was a Stepford Smiler Type B for 600 years---there are certainly hints of that----and he only tried to turn into a Type A for the last six months of his life. If this is true, the whole thing becomes even worse than it already is. He was a little kid.) If this is normal for his species, it may explain somewhat why the gender issue is a Big Deal. And why---although he is essentially the most 'female' out of the entire cast, there are some parts of Seth that are undeniably boyish in nature.
Took A Level In Badass. Unlikeliest thing ever, I know! But it happens. As this is after his canon point, however, he might never get there as far as RP is concerned. I like playing wimpy wimps.
While Rome Burns. Not really his fault as he was presumably kept in the dark while it actually happened, but during the 200 years when all the mermaids returning to Earth were systematically slaughtered---a long, thorough genocidal process that repeats itself every time they returned, unknowing of the dangers that awaited them there----Seth was on the moon, growing up happily and presumably enjoying tales of cute pink fluffies. It boggles the mind when you consider how much Sara and Miralda stressed the fact that the children (Seth and Benjamin, basically) have to become beautiful women and purify their tainted-with-human bloodline from that accursed prince so their sins could be atoned for-----while not lifting a finger to warn the mermaids and prevent the near-extinction of their race. This is made better by the fact that Seth's parents were the ones who initiated the genocide, and his attitude to it (even currently) is.........extremely detached, to say the least.
Green Eyed Monster. All the mermaid siblings are this to each other, but Seth's case was made more noticeable than the rest because it's one of the central cruxes of his dilemma. If you ask me, Teruto's case was in fact stronger and more complicated, but fans tend to ignore that in favor of the Gay Interpretation (which I think is actually part of the problem instead of a separate issue caused by fan filtering. Teruto's actions and reactions are all sort of wrong if the official explanation is all it is). Hilariously enough : Seth's eyes are canonically blue, but the author has a tendency to color them green.
Always Second Best. The main reason for him being a green-eyed monster. Seth doesn't think much of being the nicest person ever, no.
Backup Twin He is meant to be this, literally.
Second Love. He ends up being this to Shonach, and you'll root for it anyway despite seeing it coming like several miles away. The explanation for the 'second love is better than the first' clause here is that Shonach's first love was based entirely on looks and he never saw the real Benjamin behind the pretty girl, but in Seth's case it's more or less the reverse, hilariously enough. He was pretty attached to Seth from the get-go, probably because Seth's personality is that of Shonach's dream girl, but he took his time noticing the girl hidden under that boy face. To add insult to the injury, the main reason why Shonach fell in love with Benjamin was due to subliminal conditioning from dreaming about 'Seira's daughter' for six centuries, and Benjamin was that image to a tee, so he was in over his head that she's his destined love. But. As Seth is actually also 'Seira's daughter', he'd turn into exactly the same woman Shonach saw if Benjamin is out of the picture, and his personality fits her better than B's did. So who did Shonach actually see? Was it just an epic case of mistaken identity where he's supposed to have fallen in love with Seth all along and was just tested by fate? Was it the witch's curse? To add even more insult to the injury, you could make a strong argument that Shonach actually preferred the Boy!Seth that he fell in love with to the Girl!Seth that he thought he loved, although it didn't matter by then anyway. Destined/manipulated or not, they somehow managed to fall in love out of their own accord. There's a bit of subversion here, of course, in that the second love doesn't negate the first's importance, much to Seth's chagrin.
True Beauty is On the Inside. Played straight to hilarious degrees.
Our Love is Different. Subverted. He notices Shonach's Casanova tendencies and snarks about it all the time. Shonach's niceness to him, however, is really genuine person-to-person niceness, as for most of the story he's not even considered a potential romance target....and Seth just kind of never really notices it. He even denies being treated differently from the rest of the world by Shonach when it's pointed out to him.
Stay With Me Until I Die. Subverted in that he actually doesn't say it out loud due to knowing that the one he wants to say it to just doesn't see him that way, but he certainly hinted at it. It's just his greatest unspoken wish, one that remains as his greatest impetus even in this RP. Subverted again in that it's actually pretty mutual, only Shonach also has this thing with Dying Alone and they both believe that to demand something more would be entirely wrong (for different reasons), neither of them said a thing and were prepared to never see each other again as Seth left for Russia. Even when Shonach came over (to rescue Seth, not that he needed rescuing) and they both spent their last days together, neither of them realize that this trope is each other's wish. Ow. So that makes for an Esoteric Happy Ending when (spoilers) Seth's 'good' ending (the canon has two endings, the reader gets to pick) was a Someone to Remember Him By ending instead, with possibly one of the most heart-rendingly subverted Dying Declaration of Love and Never Got to Say Goodbye that I've ever read. It has all the suggestions of a good end as the genre allows, complete with The Power of Love-induced miracles (in that he's allowed to live without wrecking the Earth) and part of the audience treats his getting a kid as a good thing. But when you think of how many times he actually mentioned, thought, and in all ways implied that he only wanted to lay eggs in context of his love for Shonach and what he really wants is to spend every last minute of his life with his beloved---coupled with the fact that he never contacted Benjamin again, and the fact that breeding females cannot ever commit suicide---you wonder if he would've chosen this ending, if he could. I personally see him spending that five years raising Teruto Junior with a billion tons of Survivor's Guilt, and am glad that he didn't have to endure it for longer than that. You know, for a person with this as a True Wish (tm), it kind of sucks that everyone he loves find it necessary to die so he could live a little bit longer. Teruto, Shonach, I'm looking at you two.
Be Careful What You Wish For. Played pretty damned straight, with the clause that what he wished for wasn't his true wish, by the end. Interestingly, (spoilers) Shonach was the one who ended up with Seth's true wish, to die in the arms of the person he loves, and he seems pretty happy about it. Seth ended up with Shonach's true wish, too, to lay eggs with the person he loves. It's unknown whether Seth is happy about the deal, but at least Teruto Jr. (this is because Seth named the child 'Teruto' after his favourite brother, not that Teruto/Gill knocked him up or anything) seems like a very loved child, so at least it's a World Half Full.
Hermaphrodite. Played straight to the logical conclusion, very seriously. Has been known to break the brain of readers who obligated the questions posited by canon and think really hard about it. Seriously, the sexuality of mermaids is really really weird when you think hard about it.
Our Mermaids Are Different, now that we're on the subject. When you think of mermaids, you don't think of sixteen-year-old boys in white tuxedos who turn into salmon-like Space Whales (but only, you know, salmon-sized) who swim faster than light, do you? By the way, that suit doesn't need changing. Tuxedos are obviously the new mermaid's tail. (It makes you wonder, considering that Seth was born in the 14th century : where did the tux come from? And also : if they have legs anyway, what the hell did Seira sacrificed her voice for? Is it related to the above point? Does she need extra gear to mate with a human? Gah!)
Birth Death Juxtaposition. The entire mermaid race lives on this, but his case is pretty special.
We Are As Mayflies. Despite the fact that the typical mermaid lifespan is 200-800 years, and the fact that Seth has lived longer than some countries, you still get the impression that his time (and all mermaids) is short-lived and fleeting. This is probably because they spent forever in 'naiad' stage and turned into adults only to mate and die. Probably is not helped by how, though the other mermaids got to live out teenagerhood and all the cool things on other planets and see the wide universe, Seth was stuck in a cave on the moon until recently. He's never even seen an ocean! He cries at seeing one, he's never had friends, never lived life to the fullest. It adds to the effect, really.
Meaningful Name. 'Seth' was, according to Google, the name of Adam and Eve's third son, 'appointed to replace Abel, after he is killed by Cain'. While this seems like an inversion at first, this is played pretty straight in several ways. He is Benjamin's genetic replacement in case she dies, after all, and Teruto (Cain) only spent 13 volumes trying to kill Benjamin off. Also : in Japanese, he's written 「セツ」 or 'Setsu', which is the on-yomi reading for 'snow'. Guess what substance he is compared to the most in the story. And also, there's a saying in whatever remains of J-Fandom that 'Setsu stands for Setsunai' (「セツ」は「切ない」の「せつ」), which, if you know the the meaning thereof, is probably the most accurate one-phrase description of Seth ever.
All Love Is Unrequited. Is one of the points of the manga. I mean. Er. EVERYBODY seriously knows what everybody is feeling toward each other except maybe Art because he can't wrap his head around the concept that Hot Girl = Cute Shotaro Boy. And Rita because she just doesn't know, though she sort of guessed that this 'Seth' person was someone very special to Gill-sama. But it goes like this. Rita loves Gill/Teruto, who loves Seth, who loves Shonach, who loves Benjamin, who loves Art, who loves Jimmy (not in that way, but it totally counts). Due to one reason or the other, none of this goes anywhere until near the end.
A Tragedy of Impulsiveness. Subverted and played straight. The root causes of all tragedies is Seth thinking too much. The events that lead to them is because of Seth thinking too little. The kid obviously needs a middle ground.
Dreaming of Things To Come. Not as big a part of his plot as Benjamin, due to him giving less of a shit about humans than she did, and due to the relevant decision not being in his hands. It is still a part of his plot, however, and he agonized over it quite a fair deal.
Power Trio. Teruto is the cold logical one, Benjamin is the nice humanistic one and Seth is the rational intuitive one. Back when the three were actually living together, Seth actually played balancer and peacemaker between his two siblings, and there it's played straight. As Benjamin gets amnesia and Teruto disappears into his World Destroyer Plan, however, this trope is inverted to hell and back and balancing the opposites become the last thing that mattered in his plot. Being thrown out of his original role basically made Seth feel lost and useless, however, and left him emotionally wide open for all sorts of nasty things. Like falling in love. Which also is a clause in world-ending curse, mind.
The Hecate Sisters. Teruto is the sharp-witted crone (although he talks like a guy) and actually has dealings with a real witch, Benjamin is the blonde and naive ditz, and Seth is the incredibly maternal mother.
Cain and Abel It started as sibling rivalry (and it's sibling rivalry like whoa, too), but there comes a point where everybody wants to kill Benjamin.
Middle Child Syndrome. Subverted. When they were still family, it's Seth who was treated more like the youngest child by Teruto, while Benjamin was treated like the youngest child by their parents and both felt pretty alienated from each other. The real reason for his Middle Child Syndrome is the fact that he has always been sickly and know that he'll never succeed----unlike the Oldest Child, who usually at least succeeds at being himself, and the Youngest Child, who succeeds because they do....
Youngest Child Wins ....Guess which child Seth is? No, he's not youngest.
Big Brother Worship Seth adores Teruto, however. I don't know if this is better or worse by the fact that he knows full well what Teruto's faults are, if his dying requests are any judge. Oh, he's envious like whoa, but there's no question that for 600 years, Teruto was his everything. He even sort of half-stated that he might not have an eye for Shonach if Teruto is still around. It's probably because in that case he would show his emotional vulnerability to nobody else except his twin, but still.
Living Emotional Crutch to Teruto, in return.
Different as Night and Day for Seth and Teruto, although they seemed to be Creepy Twins at first. They, uh, actually have Twin Telepathy and Wonder Twin Powers, too, and the Twin Banter is the only times you'd ever find Seth talking in a really masculine way (not as much as Teruto's use of 'ore', though) and actually relax. And they both end up with different versions of Angsty Surviving Twin. There's also a healthy dose of Twincest, although how much of this is actual and how much of is just mermaids being weird is kind of up in the air. It is shown, however, that they're not actually different as night and day after all. It's just that each has placed the other as their ideal, and then ends up feeling super inadequate and then perpetuates the difference even more. Why do I say twins even though they're triplets? Because, really, as far as Seth and Teruto are concerned, they're pretty much twins and Benjamin is the younger sibling. B really was the odd man out, which makes his situation Fridge Tragedy when you think about it; is there any surprise that B jumped at familial warm the moment he runs into one?
Heroic BSOD. This happened when Teruto disappeared, even if it's handled somewhat subtly. He got better after meeting Shonach and getting reminded that the world still exists. Predictably, he falls in love and it rolls downhill from there. (This is remarkable, as Shonach is actually kind of a cold superficial jerk, albeit a very devoted and romantic one, and the lines that Seth originally interpreted as kindness was never meant that way-----and he realized they were never meant that way. It actually ties into something else in his character, but that one is rare enough that it's not a trope.)
Apocalypse Maiden. All three of the siblings, in different ways. In Benjamin's case, it's because Destiny Says So. In Seth's case, it's because of Black Magic. In Teruto's case, it's because he's a Woobie Destroyer of Worlds. Nevermind that Apocalypse, in this case, is more or less a Space Whale Aesop. I mean. This is a story about three actual Space Whales, man.
Beta Couple, Breakout Character. While it's obvious that Shonach/Seth was intended to happen since the story direction changes in vol.2, the fan popularity and author investment (screentime) in the pair eventually eclipsed the main couple itself. By the ending, they're basically treated as the main 'romance', and way back when I read it there was a saying going round that 'Moon Child is a manga you read for the secondary pairing'. I'm not even kidding. I appreciate Art a whole lot better now, but it's seriously like this : 90% of the fandom ship Shonach/Seth, 9% ship Teruto/Seth and 1% ship Art/Benjamin. There were more Art/Benjamin shippers when it was running, if author commentary is to be believed, but nobody really remembered them 20 years later.
Jumping the Gender Barrier. When you've been rooting for it for pretty much the whole story but you can't decide what to feel when it finally happens, I tip my hat off to the author and ask sweetly if I can burn her house.
Yaoi Guys. Consequence of the above, and visually. I confess that I seriously can't see Seth as a male character, despite the fact that he is vastly more easily understood as a boy and I do treat/write him as one, and so this doesn't really apply to how I interprete him. His relationship, as far as I'm concerned, is het, and I treat him as a bit of a transgendered person. See this picture? If you haven't read the series, that's a bona fide BL picture. If you've finished reading, then they're the most star-crossed hero and heroine in the history of ever. Seriously. You will, by the end, see Seth as a heroine instead of as a Yaoi Guy. But it's there, as a shoujo manga trope. And there is still debate about this in Japan---whether to consider Shonach/Seth BL or het---23 years after the publication. It's not a straight case of genderbent because Seth isn't really a girl either, and not just in looks. But the trope, it's there.
Your Days Are Numbered. Ultimately, the reason behind everything he does.