Like my last foray into preachin' to the choir, the following was not inspired by any singular event, story or person.
I would like to talk about Leon, but that is pretty much a given. Over the years, I've sort of unconsciously latched onto him and spend the majority of my own Kingdom Hearts stories peering over his shoulder and occasionally poking around in his brain. But as a disclaimer, I have to say that I in no way believe my interpretation to be the One True Interpretation and while I will likely use some of my own examples below, I will also use other examples and draw heavily from canon, because, y'know, that's what it's there for.
Originally, there were no plans to put Final Fantasy characters in Kingdom Hearts. Square was just making a game for another company, which has happened several times before and will probably happen again. However, perhaps in the first of many exceptionally lazy moments spanning the franchise, it was concluded that creating/designing a bunch of (important) NPCs was going to be a pain. And hey, everyone loves Final Fantasy cameos, right? AMIRITE?
You don't actually have to answer - whatever your opinion is, it's a good one for a good reason. I know. I understand.
Leon is, of course, a guy named Squall. He was referred to as Squall in media information (likely to avoid confusion since, y'know, the FFVIII was still fairly recent at the time of KH's original release) and even on his own original character sheet.
While any non-gameplay information in strategy guides should be held to certain scrutiny, in the original KH guidebook, Leon is referred to as Squall Leonhart, SeeD graduate and squad captain from Balamb, though his own in-game and future-series backstory places him in Hollow Bastion/Radiant Garden.
So who is he?
That is your choice. I've seen excellent stories that use one origin or the other and do it quite well. The 'FFVIII' origin was certainly more common before the sequels but there's no reason not to use it if an author can make it believable.
His name is Squall Leonhart, aka 'Leon'.
He claims to have changed his name to disconnect from his past, when he couldn't save his hometown when the heartless came. Remember that - we'll come back to it. But for now, his name - Yuffie still calls him 'Squall' on occasion, which almost suggests that his name change was fairly recent to Sora's arrival in Traverse Town. After all, nicknames catch on fairly quickly and Yuffie, as the youngest of the survivors who quite likely barely knew Leon beforehand, should have been the quickest to adapt.
Think about that a little. How does Leon really feel when someone calls him 'Squall'? Does it immediately take him rushing back to the night his world was lost? Or does it mildly irritate him because he's already changed his driver's license and would get on with his life? Or is there just a mental sigh, a correction and life goes on? There's no right answer, but there needs to be a conscious decision when writing him and having a character mis-identify him.
And, as a sub-note, how does he react when Hollow Bastion/Radiant Garden is restored? Surely, most of the recreated residents would remember him as 'Squall' and refer to him as such.
This guy is Squall - from Final Fantasy VIII. I've seen it asked repeatedly if playing FFVIII will help with writing Leon. I've seen it stated repeatedly that Leon is not FFVIII!Squall so quit writing him as such. Again, this goes both ways and needs to be a conscious decision. Leon's KH character development mirrors Squall's development in FFVIII - he grows from a solitary, brooding teenager into an almost-friendly squad-leader who genuinely cares about his friends and teammates. Do you need to play FFVIII to be able to write Leon? Probably not. But keep him in mind, because he's an interesting character and some of his mannerisms and details can be used to help fill in some of Leon's blanks.
Interestingly, I found myself writing Leon and KH fiction several months before I was able to play FFVIII and was shocked to discover that in-game, Squall did several things that I'd subconsciously assigned to Leon because they were things a character fitting Leon's archetype might do. Go figure.
Using the canon established throughout the three KH games, Hollow Bastion/Radiant Garden fell about nine years before the first game. Leon is 25 at the beginning of the first game. That means he was about SIXTEEN when he lost his home. Sure, that's a great age for aspiring game heroes and mecha pilots, however, assuming that Leon did not already have several years of training as an aspiring hero or mecha pilot, what sixteen year old blames HIMSELF for the loss of his hometown, especially when it's fairly common knowledge (to him) that the witch Maleficent was involved?
So why does he blame himself so much? Why did he expect to have any bearing on the outcome? Did he at some point assist Maleficent either consciously or unwittingly? Was he part of a last-ditch reserve guard? Did he falter at a crucial time?
Leon is a character who has a deep inner motivation for everything he does. He's in charge of defending Traverse Town from the heartless and spends most of his spare time training or researching. He's later in charge of the Hollow Bastion Restoration Committee - again referring to the deeply-rooted guilt he carries over what happened.
Again, make some conscious decisions. Leon isn't the sort of character who simply does things on a whim - he has a greater motivation for nearly everything he does and can probably be a little single-minded in that regard, especially in KH1. He is a man on a mission even though the mission might border on hopeless/unclear at times.
Also, as a side-note, how does Leon feel about Sora being the keyblade's chosen? Do you think that maybe he'd sort of envisioned himself as the potential keyblade wielder?
Leon is a smart man. He is a strong man. He is likely a competent strategist and has a firm understanding of command chains, logistics, etc. He is not in charge of two very important groups for no reason. However, he is aware of his faults and would not deliberately use or manipulate people for his own gain.
Following that, Leon cares deeply for people, even if he initially won't admit it. He's torn up over losing his hometown, not specifically his family or a specific group. He has been in a family-unit with Cid, Aerith and Yuffie for about nine years and likely has unique relationships with each of them, however, he defends ALL of Traverse Town and it isn't likely that he does that just because he really likes the noodles and tea at the cafe or doesn't want the shoe shop to close because the owner has been heartless-ized. However, it's not something Leon will necessarily admit to without a whole lot of lead-in.
Before the fall of Hollow Bastion/Radiant Garden, Leon is pretty much a blank slate. Did he live with his mother (Raine) and father (Laguna)? One of the two? Why? He has his mother's last name and uses a version of it for his second name - does he have very distinct feelings regarding her? Unlike in FFVIII, he should remember his childhood. Was he a completely different personality than what he became? If he was as seemingly distanced and stoic as a teen, why was he so concerned about losing his home? When he lost his home, who particularly did he lose and how does he feel about that? Was there an Ellone character in his life? Are any of these relatives returned to him when Radiant Garden is restored? How does he handle having his family back after so many years of Cid, Aerith and Yuffie being his family? Don't forget, he was called 'Squall' back then. He wouldn't have used 'Leon' even as a nickname, because part of his using Leon later in life was as a 'disconnect' from his past. He wouldn't choose something he'd necessarily answered to before, just something that was close enough to be a lingering reminder.
There are endless variations on this theme - the main point being 'think about it'. Creating an original family for Leon is acceptable; Leon is not Squall from FFVIII - but don't expect everyone to like it.
Just as interesting as the inner-workings of Leon's character is how other characters view Leon. Since Kingdom Hearts follows over Sora's shoulder, there aren't many opportunities to get different viewpoints on Leon.
Jiminy writes of him: "A swordsman who carries a gunblade. His real name is Squall Leonhart. he took the name Leon to distance himself from his past self -- a man who stood helpless as the Heartless invaded his home.
For some time he took refuge in a different town, but thanks in part to Sora's help, Leon was able to reclaim this [Hollow Bastion/Radiant Garden], his hometown. Now he works tirelessly towards its restoration."
In Chain of Memories, even though the Leon presented isn't real, he's even more useful as a representation of Sora's thoughts on Leon. CoM's Leon is Sora's memory of Leon and provides quite a bit of information.
Sora's Leon has a sense of humor, even if it is a bit sarcastic. Sora's Leon is a teacher and takes the time to explain the game's battle system to Sora. Sora's Leon wants to believe and wants to find an explanation for events occurring.
Sora is probably not the only person that Leon has ever stopped to help. So chances are, other characters view Leon in a similar way, depending on their relationship with him and when they met him. Even random residents of Traverse Town may think 'That Leon is a good guy. He keeps us safe and the other day when I said hello to him, he stopped to ask how I was. I mentioned I had a light out in the kitchen and five minutes later he had a ladder and was fixing it. He's a bit of a loner, I guess, but he's not a bad guy, all 'round.'
I realize that some of you apparently played a different game than I did. Honestly, if it's well-written, I really don't care who you pair Leon with because a good writer can suspend disbelief. However, Leon is not going to jump into any relationship without a whole lot mismatched baggage.
I have no intention of going through every possible Leon-pairing. It basically comes down to 'please think about his motivations and how he'd react in any given situation'. Would Leon date Yuffie, who he has probably helped care for since she was seven? Would he feel awkward about their age difference? With Sora, would there be a distinct power-balance issue? With Riku, would their deep-rooted guilt-complexes collide into a sea of angst? Etc.
PWPs do not need excessive characterization - hence the term 'porn without plot/plot? what plot?'. That doesn't mean that just slapping character names on humping bodies is enough. A few little details go a long way and can even make the scene more interesting. Especially if they're the right, well-planned details. There's no lack of information available or interesting scenarios to pose.
How does Leon regard relationships, anyway? Has he ever been in one? It's lightly hinted at that a Rinoa character exists in the KH universe, but was she a girlfriend? A neighbor? Was it a one-sided crush? Was it something from when Leon was sixteen that has absolutely nothing to do with his feelings at twenty-six? More things to consider, really.
The canon material is really fun - work with it! It's really quite versatile and has room to explore a multitude of options.
However, there are some things that Leon probably wouldn't do:
Leon is probably not a rapist. Leon has some very obvious power issues and a very deep well of frustration. However he'd probably work it out by training or smashing his fist through a wall or just by walking away from any given situation where his life wasn't in danger.
Leon probably wouldn't think of his relationship partner as 'his property'. Remember, Leon genuinely seems to care about everyone. He probably wouldn't even joke about it.
Leon probably wouldn't think about Insert Character Here with all sorts of flowery adjectives. He is literate and likely well-read. However he has never shown any inclination towards the poetic or purple. When he mentions that the sunset isn't red but instead a wash of maroon and rose descending over the peaked grey-slate rooftops, I'll not wince when he talks about how pretty someone's eyes are.
Leon probably isn't a drug addict. Sure, he's full of angst and pain and guilt but he's already decided that he wants to overcome that and is devoted to making himself better for the next time around. He's probably not a cutter for the exact same reasons.
In closing, Leon is an interesting non-main character. He has a full characterization packed with backstory and motivation though there are still a thousand opportunities to be explored. He fits well into many situations and can fulfill a variety of roles, assuming smart choices are made.
(All factual information used above is as correct as I can make it, paraphrased from memory unless quoted and can probably be attributed but it might take awhile because some sources are print and I'd have to find them first. Graphics are from the following sources: E3 KH1 art - rpgamer.com, Squall art - unknown, CoM sprites credited on graphic, KH2: Yaoi Mix manip by yours truly.)