lengthy stuff
Psychology:
The short answer to this is: Tsuzuki is a darling, lovable MESS.
And now for the long answer.
On the surface, Tsuzuki is the office idiot. A klutz who unintentionally destroys things on a regular basis (as small as a coffee machine or as large as the entire Juuouchou library), he's sweet and cuddly and friendly, so most everyone has a hard time disliking him. He's also a glutton who eats so much so quickly that he tends to get the aim wrong from the fork to his mouth, and he can smell any sweet within the nearest mile. He's a drunk and a lazy slob, who often comes into work late (most of the time slightly hungover), and with his clothes a rumpled mess. He avoids paperwork like the plague, and is content to stay in the backwoods where cases are few and easy instead of doing actual work. If anything, his faults make him more endearing, and everyone likes him as a sort of haphazard pet as well as a good friend. He's far too sensitive. He can't stand to hurt anyone or anything and has no respect for himself. And, apparently, his bouncy, happy, scatterbrain personality covers a huge well of self-hatred and suicidal tendencies.
Tsuzuki desperately wants everyone to think he's simple, one-dimensional. He's very good at keeping up the act (though everyone suspects differently, but politely says nothing), but he sure tries hard. Tsuzuki isn't stupid or lazy; he carries more guilt and remorse than one person should ever have to feel, and he will never get used to the cases that require them to take a human life. He clearly believes that he's worthless, and yet he craves acceptance and love, a dichotomy that drives him to present himself as the office idiot, someone funny and friendly and likable. Even when insulted and pushed away, this need for approval keeps him coming back and never hating the offender in the slightest for even a moment.
But it's not all a front, either. Tsuzuki is genuinely sweet and kind, more than capable of easy, frank affection. He does like sunny mornings and ice cream and puppies, and he takes joy in little things. He's gentle and patient, and pretty intelligent, but in a casual way that he easily dismisses (and so does everyone else). He's absolutely honest in this one area, if nowhere else: he would never betray a trust, nor hurt a loved one. Tsuzuki always cares, no matter how it hurts him. He's forever doubting and uncertain - especially about himself - and needs unflinching honesty and steadiness since he can't manage it for himself.
Tsuzuki is also very bad at keeping his emotions in check, and often lets them cloud his judgement. He may seem almost frightening and calm when angered, but he comes out with all guns blazing and not much thought for anything else. He does his best to jump in and fix everything for everyone else and try to not let get anyone hurt in the process, but he's so afraid of failure that he often hesitates. He's so sympathetic to others' pain that he loses objectivity and sight of the big picture at times. He also doesn't seem to know the meaning of a "happy medium"; everything he does is all or nothing, extreme and passionate.
Tsuzuki's a hypocrite in weighing his own worth against others; everyone else comes first 99% of the time, but once he loses control of his sanity, he runs to destroy himself without thinking of who he might be hurting in the process. He will always put someone else before himself and throw himself into danger in order to protect someone else, but if someone turns that logic towards him, he blatantly and gently denies it, unable to believe he could be important to anyone and worth saving, ever.
At the time of the series, Tsuzuki has gotten a lot stronger emotionally since he first started as a shinigami. This is not a positive thing, because he had to grow strong and silent about his emotional ills out of necessity, not out of growth and self-absolving. He realized what a burden he was putting on everyone around him, and rather than bother anyone with his "less than worthy" problems, he simply shut up and closed himself off, rarely leaning on anyone except when cataclysmically vulnerable.
History:
The biggest problem with Tsuzuki's life-before-death is that not much is known about it. There are clues scattered throughout canon, but still not enough to paint a clear picture. Much is left to the imagination.
Tsuzuki was born in the year 1900, and that's the most solid fact known about his early life. His family is never mentioned, except that he had a sister he loved very much, named Ruka. She's the one who taught him gardening, dancing, and cooking. As a child, he was often physically bullied by his peers for having purple eyes, the mark of demon blood. He never fought back, only running while screaming apologies just for being alive. At age 18, something tragic happened, and Tsuzuki mentally imploded into insanity. Whether he killed someone or many people or just found the demon blood allegations to be true, he couldn't bear it and lost his mind. He spent the next seven years in a hospital, where he continued to live despite being penniless, not to mention not eating, sleeping, or drinking. He attempted suicide countless times, but it seems the shinigami healing ability was one he possessed long before death; his attempts always failed simply because the wounds would heal almost immediately. It wasn't until January 1926 that he finally got "lucky".
(Personally, I think Tsuzuki was in love with Ruka - it's hinted at in the manga, that he had a "forbidden" love, and Ruka is the only person from his life that we ever hear about. I think it's also possible that he killed her by accident. Whether just her or her included in a group of people, who can say. But that's what I think might have happened.)
Though we don't see the first seventy years of Tsuzuki's "life" as shinigami, we know that he used to be highly unstable and would often fall into horrible depressions if he failed at a job, or should someone innocent die, or any number of triggers. Tsuzuki has been a shinigami so long either because he can't forgive himself and can't move on, OR his crimes were so horrific that his penance hasn't been completed yet. He was never able to keep a partner for longer than two years; his partners often wanted to help Tsuzuki, but in the end could never deal with the depth of his pain.
This was until 1996, when a new shinigami entered the fold. Hisoka Kurosaki was only 16 when he died, and is a fierce opposite to Tsuzuki in many ways. Aloof, misanthropistic, practical, intelligent, and very short-tempered, Hisoka was furious to find he got the shinigami with the department's worst reputation as a partner. Slowly, Hisoka sees a lot of things the others in the office miss - Tsuzuki's work ethic, for one thing, his intelligence and kindness being another. Hisoka grew up without any kindness at all and has a hard time knowing how to react to Tsuzuki, so he tries to push him away. Unfortunately, this ends up getting Hisoka kidnapped. The perpetrator behind their first case together is Muraki, a doctor from Tokyo who mysteriously has many spiritual abilities despite still being alive. Muraki is actually the one who killed Hisoka, and he kidnaps the boy in order to lure Tsuzuki out, knowing that Tsuzuki will run to save Hisoka, even knowing it's a trap. Which is exactly what happens. Tsuzuki and Muraki duke it out with shikigami, but Muraki sends a fatal blow towards a weakened Hisoka - which Tsuzuki takes instead. It shatters his spine, alone with several organs, probably. In desperation, Tsuzuki uses Hisoka's empathy to his advantage and asks if he could channel himself through Hisoka in order to save them both. So they sorta meld minds, and thus are able to escape the burning building more or less alive. Unfortunately, Muraki was also able to. It takes some time, but Hisoka finally decides not to ask for a new partner after this first case of theirs is finished. Tsuzuki selflessly risked his life to save Hisoka from being kidnapped by Muraki, and he's never been shown such kindness.
Still, the two are very different and have yet to know their similarities. In the next case, a boy named Hijiri has unfortunately been given a new cornea - only to find it has a contract with a demon from the previous owner written on it. Tsuzuki promises to protect Hijiri at all costs, but Hisoka scolds him for making promises he might not be able to keep. Unfortunately, this ends up partly true - Tsuzuki becomes possessed by the demon that they're fighting, who uses Tsuzuki's body to kill Hisoka (disguised as Hijiri), and inflitrate and destroy Juuouchou using Tsuzuki's shikigami. A lot of damamge is done before Hisoka is able to perform a spirit capture. It fails the first time because Hijiri interrupts Hisoka, saying that Tsuzuki can't take that kind of pressure on himself. But Hisoka says they have to trust Tsuzuki's strength. It finally works the second time, and a crack is made in the endless void that the demon created in Tsuzuki's mind to keep him captured. He breaks free, but has destroyed much of Juouchou in the meantime. No one blames him except himself. Hijiri thanks Tsuzuki profusely for everything he's done, and promised he will play his violin for the rest of his life in tribute to them. Hisoka also comforts Tsuzuki in the only way he can manage; with blunt honesty. He admits that he was wrong about Tsuzuki's intentions and now understands that when Tsuzuki makes a promise, he intends to keep it.
Next case! There have been Japanese people mysteriously continuing to live when they should die, and lots of people from Hong Kong dying long before their time. The time periods coincide with one another, and also with a cruise liner called the Queen Camellia that goes between Nagasaki and Hong Kong. Tsuzuki is disguised as a poker dealer (practically a genius one, at that) in order to hear what the passengers have to say, while Hisoka poses as the son of a businessman in order to get close to the higher-ups, including the owner of the ship, Kakyoin. Tsuzuki hears rumors of a ghost that roams the halls of the VIP suites at night, while Hisoka runs into Kakyoin's daughter, Tsubaki. They also run into a familiar and unwelcome face - Muraki, who's there as the doctor for Tsubaki, as she has a weak heart. Tsuzuki immediately accuses Muraki of the mysterious deaths, but Muraki merely laughs this off. Later that night, a politician is killed; first poisoned to death, then his heart ripped out. He's found with a tarot card that means "beginning." Again, Tsuzuki and Hisoka point the finger at Muraki. But when he dies the second night, along with Kakyoin, there's nothing but confusion. Again, tarot cards are left as signs with the bodies. Tsuzuki inadvertently finds a microchip hidden in the politician's ring, where a map of the ship points to a blank space within one of the storage units. Tsuzuki finds Muraki there, alive and well, immune to the poison the murderer uses because he builds an immunity by drinking a mix of poisons by choice, so he was in suspended animation. Lovely. After discovering the last body, Tsuzuki and Hisoka return to the storage unit to find a state-of-the-art operating room, and Tsubaki. Tsubaki claims that her name is Irene, and she was killed in order for her heart to be given to Tsubaki in a transplant to save her life. But this is hypnosis. It turns out that Muraki WAS the killer after all; he orchestrated the operations and connection of victim donors and receivers. He also hypnotized Tsubaki through her anti-rejection medication, causing her to believe she was Irene's spirit still living in Tsubaki and killing others. Now that the whole plan is exposed, Muraki shoots Tsubaki and sets off a bomb. Tsubaki begs Hisoka, with whom she had gotten somewhat close to, to shoot her and put her out of her misery. He does so, very unwillingly. Tsubaki tells him not to worry; that there is someone nearby who understands and loves him no matter what he does and who will always be there for him. Tsuzuki shows up at this point in a helicopter piloted by Tatsumi, and they escape the burning wreckage.
And now for the final case before his canon. Though it's out of their jurisdiction, Tsuzuki and Hisoka were asked to come to Kyoto for a mysterious and brutal series of serial murders. A certain piece of evidence, a lock of silver hair, was found clutched in one of the victim's hands. A purposeful message left by Muraki to bring Tsuzuki to him again. They run into him one night, still holding the corpse of someone he had just killed. Muraki blackmailed Tsuzuki into following him (otherwise Hisoka, on whom he had a deathgrip, would get hurt) to a restaurant. Tsuzuki refused to ouch the food, so Muraki continued to goad him. When Tsuzuki lost control and threw a punch, Muraki easily got him in a stranglehold, and casually remarked on how Tsuzuki wears his watch on his right wrist instead of his left, insinuating that Tsuzuki was hiding something. Muraki left Tsuzuki to emo about the fact that yes, Tsuzuki died by committing suicide. Hisoka tried comforting him, but Tsuzuki shut him off and said nothing about himself. They soon had a lead on Muraki; a professor at a local university who specialized in clone research. But Muraki is helping with that clone research (he's killing all those women for cuttings of their hair for DNA)...and he told the professor about Tsuzuki. Muraki knew about Tsuzuki because of records he found of his grandfather's, who was also a doctor. His grandfather had a patient in his hospital that he let stay for free because he lived for seven years without food, water, or sleep. Not to mention numerous suicide attempts refused to work because of rapid healing ability. And that was Tsuzuki. He finally succeeded in committing suicide, but the fact that this person has "life" again was motivation for the professor to beg for Muraki to use him as a "sample". Muraki agreed, but said he'd need the professor's cooperation in capturing him. Muraki had cornered two high school girls who had run into Tsuzuki and Hisoka and killed one of them, saying it was her punishment for seeing shinigami. Tsuzuki came too late and was unable to act, paralyzed with horror and guilt as Muraki blamed him for the girl's death. Tatsumi came in time to save them and the other girl, but now muraki's out to finish the job. Tsuzuki and the others join the girl's school as faculty (and a student, in Hisoka's case) in order to keep an eye on her. But the professor and Muraki trick her out of the safety of the watchful eyes. Muraki once again blackmailed Tsuzuki to accompany him to a kabuki play in return for her safety. Muraki goads Tsuzuki again, this time revealing this he has his grandfather's files from when Tsuzuki was alive, and that he had analyzed Tsuzuki's DNA - it isn't human. Tsuzuki is so upset about this that after Muraki releases him, he later drags Hisoka to a bar and gets very drunk. He confides in Hisoka that he may not be human, then runs outside and tries to stab his eyes out, hating them for being marks of his demon blood with their purple color. Of course, they heal quickly due to the shinigami healing. Hisoka grabbed his collar and snapped him out of his sudden self-harming spree, assuring him that he IS human and that he'd made the choice to always stay with Tsuzuki. They find the girl Muraki's targeting not long after, and take her to Juouchou for medical treatment. Muraki's placed a centipede demon within her, and releases it once most of Juouchou has fallen asleep. Tsuzuki was keeping watch over her, and though Juouchou is being attacked, he refuses to fight her. His shikigami Suzaku is protective of him and destroys the centipede in order to protect him, but it's the last thing he wants. Another innocent person died because of him, so he thinks. And thus he loses his mind. He implodes deep into his mind and Suzaku protects him from anyone who comes near, even his friends. Muraki materializes and steals Tsuzuki away. Muraki's taken Tsuzuki to his lab (see the NPC section at the end of the Reality Description) in order to decapitate him. Tsuzuki continues to be lost inside his mind, but hedoes have some moments of lucidity. It was one of those moments where he takes advantage of Muraki's dropped guard - and stabs him. He then recites the incantation to call his shikigami Touda - a black serpent dragon who uses black fire, which is one of the few methods that can guaranteed kill a shinigami. He intends to kill himself, and Muraki with him. By the time Hisoka and the others reach the lab, it's ablaze. Hisoka dives in without thinking because he refuses to let Tsuzuki kill himself. Hisoka finally reaches his partner, but Tsuzuki refuses to be saved. He says that he's tired of living, that he's killed too many people and he shouldn't have existed in the first place (and other such emo things), and Hisoka hugs him. Hisoka's crying, desperate to reach Tsuzuki. He begs Tsuzuki to live, even if the only reason would be to live for Hisoka. Because Hisoka needs him, and all the kindness he gives. Tsuzuki's shocked, but finally relents. They barely escape the inferno alive, but they've survived together.
Tsuzuki's spent the past few months since then recovering. He's a lot stronger since then, now that he has someone he feels he can rely on. He's still a bit shaky, especially on the subject of Muraki. But he's come out of that purifying fire a better person.
NPCs
The most important NPCs would probably be Hisoka and Muraki. Muraki is Tsuzuki's nemesis in the series. He's everything Tsuzuki isn't; apathetic, murderous, blatantly sexual, cold, psychopathic, selfish, and etc. Muraki is responsible for many of the incidents over the course of the series, not to mention Hisoka's rape and murder. Hisoka is the only known case of Muraki raping someone, but one of possibly hundreds of his murder victims. He's an extremely evil, insane, twisted (read: fucked up) bastard. His half-brother Saki killed their parents before heading for him, but a servant of the household shot Saki before he could do anything. Since then, Muraki has been on an insane quest to revive Saki just for the sake of killing him with his own hands. Muraki keeps Saki's head alive in an enormous laboratory, and it's for this purpose that Muraki wants Tsuzuki. Not only does he find Tsuzuki attractive and want to mentally break him (since that's how he expresses love? ack), but Muraki wants to use Tsuzuki's immortal body in order to revive Saki. In the meantime, he's killed countless people to satisfy his bloodlust.
Hisoka is Tsuzuki's partner as a shinigami. Hisoka is very pretty and small for a boy, and a strong empath who grew up in an old, traditional Japanese family. Once they found out of his ability to read others' minds and feelings, they locked him in a cell in the basement for most of his life. So he grew up not only without kindness, but he was treated with revulsion and called a monster. At 13, Hisoka was wandering the gardens one night. Under the light of the red moon, he witnessed Muraki killing someone. Muraki, of course, can't have witness...but he deemed Hisoka too beautiful to kill the same way. So Muraki raped Hisoka...while carving a curse into his body. It slowly killed Hisoka over the next three years as a terminal illness that doctors couldn't cure. The curse still appears on Hisoka's body when Muraki is "active", and probably always will until Muraki ceases to exist. So Hisoka has grown hard thanks to his many horrible experiences, but Tsuzuki wormed his way into his consciousness in a way no one else had.
major depression - it's easy to label Tsuzuki as depressed, but he's somewhat asympomatic - he still takes joy in lots of things, especially his hobby of gardening. He's not excessively lethargic, and his horrific work ethic is due to other factors, not apathy or lethargy. However, he does suffer from insufferable guilt and feelings of worthlessness, feels isolated from others, and has trouble with concentration (and the ADHD probably doesn't help). He also possibly sleeps for unnecessarily long periods of time (sleep is described as one of his favorite things and he's often late to work for oversleeping or hangovers); this doesn't feed into being lethargic because he's energetic and alert when he is awake. It's also popular fandom opinion that there are just some days where Tsuzuki is in too much despair to get out of bed and just stays there until he can gather the strength to go back out into the world. Much of this is exacerbated by his other conditions.
moral existential anxiety - Tsuzuki has attempted suicide (and succeeded, once) countless times because he doesn't just believe his existence to be worthless, but also as a detriment to the world and those around him. He believes that he brings nothing but pain and suffering, and wants to free himself from the despair of this belief, and free the world of his negative existence, because of extreme guilt and condemnation.
borderline personality disorder - One of the common factors to diagnose this condition is a narcissistic view, which Tsuzuki definitely doesn't have. However, having reading several articles, I've decided to keep this condition because it fits him perfectly minus that area, and the narcissism is a tendency and not necessarily an inevitability if one has this condition.
Tsuzuki has rapid mood swings, but they're not severe enough to be considered bipolar. He switches from bouncy/hyper to depressed very easily, and sometimes with little provocation. Many BPD mood swings are punctuated with some sort of aggression, self-harm, or substance abuse - the last two are both definitely Tsuzuki, considering his alcoholism. He's often paranoid about abandonment and hides pieces of himself he thinks are necessary to keep other from hating him, not to mention much too sensitive (easily hurt) and hyper-aware of the reactions of those around him. This is partially because he believes himself to be fundamentally bad and unworthy (another symptom, though it also factors into his other diagnoses). He has a history of failed relationships of all sorts and none of them could be considered stable during or after the fact. (Tatsumi is the most prominent example of this - he was another partner who was unable to cope with Tsuzuki's extreme instability, and at best the space between them now is tense and awkward.) Tsuzuki often switches between idealizing others and their reactions; when it's different from what he hoped/imagined, he perceives himself to be hated and turns the idealist fantasy completely around. In a typical BPD response, the BPD sufferer would threaten suicide because the reality meant their object of affection didn't care for them. This is another area where Tsuzuki doesn't follow the pattern. BUT it's not impossible to think that being so old and repeating these behaviors for so long, he's become aware that he creates these ideals but is unable to help himself. If anything, he takes it as his own personal failure and hates himself more when his ideals are crumbled. As for a self-disturbed self-image, we don't need to look further than the fact that Tsuzuki can't ascertain for himself whether he's even human or not. Though he's no longer constantly considering or attempting suicide, it's not a far stretch to imagine that he picks at himself (maybe biting the skin of his fingers, nail biting, hair pulling). Need I also mention that depression and alcoholism usually coincide with this condition, as well as impulsive behaviors like binge eating? Oh, and did I mention the dissociation? In canon, Tsuzuki admits that after losing his mind in life, he had periods of lucidity that became less frequent and lasted less time. So the inverse that he often was "outside himself" in a sense is also true.
As the final nails in the coffin, BPD sufferers usually have a history punctuated by abuse or neglect during childhood (being frequently beaten up by the neighborhood children, and lord knows what else), and then the condition was triggered due to trauma in adolescence to early adulthood (the epic big bad thing that made him lose his mind, anyone?); BPD sufferers are more likely to be victims of violence and rape later in life (Muraki targets Tsuzuki very heavily, and for very twisted reasons?)
inferiority complex - this is exceptionally true of Tsuzuki. he believes he is completely below everyone around him, and all his principles and beliefs about the world, how it works, and people are completely irrelevant to himself. For example, he believes in the best of people and that everyone is worth saving and is a priceless life, but this doesn't apply to himself.
alcoholism - it's hinted at several times in canon (moreso in the manga) that Tsuzuki drinks quite frequently and is possibly an alcoholic. This is a method of self-medication that he relies on in order to drown himself from his pain on a regular basis, apparently. Alcoholism is disputed in medical science to be a disease or not, and in canon it's pretty clear that Tsuzuki doesn't have an irrepressible dependency on it, but it sounds like he consumes large amounts frequently enough to be considered a problem. Substance abuse is also a symptom of several of the aforementioned diagnoses.
post-traumatic stress disorder - it's likely that Tsuzuki buries the memories of whatever trauma he experienced in his past because he simply can't handle the psychological strain. But when confronted with the truth, he usually goes into yet another mental breakdown. After Kyoto, Muraki gets added as a stimulus that Tsuzuki can't mentally handle - when confronted with an illusion of Muraki in the Hall of Candles, Tsuzuki becomes absolutely paralyzed with fear and is unable to speak or move.
attention deficit hyperactive disorder - Tsuzuki has an extremely difficult time keeping focus and fidgets often. He's impulsive and often doesn't think before he acts or speaks. He's extremely disorganized and can't stick to one activity for too long. Not to mention his need to play with something when standing still, and constant fidgeting when sitting.