Character

Name: Kamui
Age: 20ish
Memory Option: Full AU; at least to start.
Established Status: Yes - About two years, give or take some months. He's been busy acting in ways that align with his AU memories - actually gathering up a space pirate crew to have contacts with, even if they're not engaging in much actual space piracy in the quad at the moment, and cultivating other shady connections. He "crash landed" and his crew dispersed around the time he arrived in-universe, so he's had time to set up and establish himself by the time the game starts.
Canon: Gintama
Canon Point: Chapter 589
Citizenship: No official citizenship in the quad; formerly from planets called Kouan and Rakuyou. Has fake papers for Leith, if that's alright.
Job: Super shady space pirate.
Abilities:

☂ Boy 2 Stronk: Kamui is one of the most physically capable members of the entire Gintama cast. He is highly skilled in fighting, and has the strength, endurance, and body sense to back up the skills he's learned over the years.

☂ Evil Space Bunny: Kamui, despite all appearances is not human. Instead, he is a member of an alien race known as the Yato, which are known for their exceptional strength, agility, heightened senses and accelerated healing ability, as compared to ordinary humans. Additionally, he's considered to be one of the highest elite members of the Yato clan, with his own abilities far surpassing the average of his race. Yato are also exceptionally bloodthirsty as compared to humans and many other Amanto, a trait which Kamui also has in spades.

☂ In-universe this means: In a few words; experimental nanotechnology, courtesy of the universe. Thanks, universe. But as assisted with on the brainstorming meme with you mods; "instead of actually being an alien race, The abilities his non-human heritage lends could easily be translated to the 'verse. For example, the doctors that worked on his memory and power replacement could have also realized he has some unusual genetic variant and experimented with that a bit. This genetic condition could also explain the sensitivity to the sun and if the nanites were implanted after he was brought to the J, this could both explain the rapid healing ability, superhuman strength, speed, and appetite." Which I just quoted directly from your comment to me, because I do like that idea and I want to run with it - with the exception being that he'll believe it happened before he got to the J, as a child.

Personality:

"Weaklings like you... have no place with us."
[...]
"I've got no use for weaklings."
Kamui is fixated on being strong, first and foremost. As an extension of this, he's similarly fixated on being a Yato, and what it means to be a Yato, having a very rigid definition in his mind, with the primary traits being bloodthirsty and strong. Kagura is, in his opinion, neither of these things, and is therefore not worth his time, not worth being considered one of them, and can be more easily cast aside. Similarly, his entire goal in life is to be the strongest - not simply the strongest Yato, but the strongest person in the entire known and unknown universe, and he'll stop at nothing to reach that goal, expressing that those he feel are weaker than he is are not worth his consideration, much less his time...

"Samurai really are a blast."
...Most of the time. He feels that humans are weak, and in most cases he's right - compared to him, the average human is. But then he encountered Gintoki and, later, Takasugi who, despite both being humans, proved they met and far, far surpassed his expectations of what being human is. So, interestingly, he reclassifies them. The humans who piqued his interest are both samurai, so it's samurai who keep his interest when it comes to the human race. He thinks of them as a fun prey, because despite their weak bodies, they possess a tenacity that allows them to keep getting back up and even to defeat much, much stronger opponents. This is in no small part due to Gintoki being the protagonist, and therefore it being necessary for him to keep getting up time and time again until the bad guy goes down, but while Gintama does address the meta in such ways fairly often, Kamui himself has not, and probably will not, or else his line might have gone; "Protagonists really are a blast."

"I pay my respects with a smile when killing. Regardless of how their life was, it's appropriate to send them off with a smile, so they can die soundly. You could even say that I intend to kill when I'm smiling."
Kamui is almost always smiling. He has fun with his day-to-day life, especially when that fun involves running off directly into the sort of dangerous situations that could get him killed... but more often than not directly result in carnage en masse for whoever he has his sights on at the given moment, whoever he feels would be fun to go toe to toe with. But the sentiment that he intends to kill when he's smiling is an interesting one - he laughs about the admission soon after making it, when it scares Seita, but in every other instance he shows that he is always a half-step away from killing someone, and when he's really steeped in a difficult fight, such as against Housen, the Harusame's ambush, or against Sougo or Umibozu (and, eventually, Kagura), his smile grows even wider, bordering on manic excitement where getting in his way means almost certain death (rest in peace, Ungyou, you will probably be forgotten).

"Who's afraid of the big pitiful old man? Not me."
Kamui is very, very confident, to a reckless degree. The last time he'd seen Housen, he was stronger than him, and Kamui had considered him as master, but he doesn't hesitate to tear him down and aggravate him to the point where he doesn't hesitate to start a fight. And that fight gets interrupted, so we as the audience will never know if that confidence was misplaced bravado or not. Given his tendency to pick fights with people he is reasonably certain he can defeat but still have some chance of defeating him, it's anyone's guess as to whether or not his life would have ended there, but it is certain a large portion of Yoshiwara would have gone down with the loser. His confidence extends further, in his attempt to take on Harusame not for political gain, but simply to test himself against an alien race known for being exceptionally strong in large groups, but weak individually. Another interrupted fight. He runs headlong into danger, even when heavily injured, and seems to think of that as a fun handicap that allows him to fight weaker opponents and still test his strength against them.

"Is a weak-ass game piece like that gonna be of any use?"
For all his fixation on bettering himself, and for how smart he is in many other areas of life - particularly researching enemies, knowing who to ally himself with, and when it would be smart to bail out, knowing how to be a functioning captain of a space pirate vessel of Yato who are all, most likely, older than him - he is still not one for strategy, and does not know how to make use of anything, or anyone, he considers weak, and certainly does not understand their value in strategic situations, leaving all that up to other people, like Abuto and Takasugi. Still, while he may be flippant about their decisions, particularly in Abuto's case ("Yes, yes! Yes, yes!"), he only puts up a nominal protest, by way of expressing his own opinion, but doesn't get in the way of what he seems to simultaneously consider both above and below him.

After all... "I have no interest in killing the pathetic."

"You're the same as me, aren't you? You can't pull the wool over the same stupid eyes. You don't really care about the shogun's head, right? You just wanted to come here because you got a whiff of the smell of the blood. They're the eyes of a murderer. You can't feel alive unless you're on the battlefield surrounded by death. No matter how many villains you cut down, that thirst is not quenched. You've been looking for someone strong like me, and I've been looking for someone strong like you. Right? So relax. If you're saying that you can only cut down villains.... then, I'm one hell of a villain."
Kamui sees himself strongly in Sougo, and the assertions he makes in the heat of the moment are more telling than he'd want, or would make to someone he wasn't currently trying to rile up into a fight. He follows trouble, he follows "the smell of blood," which he's never hidden before, but this indicates it's a conscious decision to do so. He knows the thirst for blood can only be quenched with a tough opponent one who, in all likelihood, will inflict serious injury and exhaustion, quieting the bloodlust because the physical body is incapable of keeping up. Most telling; however, is the assertion that Sougo - and, therefore, Kamui himself - cannot feel alive when he's not on the battlefield, that the adrenaline rush, and the hunt for the next rush in the interim, is the only thing keeping them going. A strange assertion, for one who appears so cheerful all the time, but an indication that there is something deeper under the surface that even he is careful to not let show.

"I don't have anything I want, nor do I have a reason to turn back. I can't see what lies ahead. Everything blurs together into a new and everlasting battlefield. And it's there I feel most at home. I'll keep advancing, until I'm stronger than anyone or anything, as what I needed to protect is no longer here."
Kamui, at a base level, lives in the moment. He has motivations, but they're vague and short-sighted. He wishes to become the strongest, but he doesn't really know what that means, and he knows that to accomplish this he has to fight and fight and keep fighting, so he's built his world up to be a hazy battlefield. He has to keep moving forward, because he can't go back, and his motivations have evolved with him, have simplified from "become stronger to protect mammy and Kagura" to just "become the strongest," because he's already failed. Umibozu left. He wasn't strong enough to protect his family, and put that task on little Kamui's shoulders, nearly killing him when he came back and Kamui wasn't strong enough to kill him first. He was the man of the house, at the time, and fought and fought to get stronger to protect his family, all while hiding whenever he got hurt, putting on a smile for their sick mother. But you can't punch illness in the face, so he was never strong enough to protect what he needed to, so his goals have shifted, because he doesn't really have much else.

AU History:

As the firstborn of a small family living on a slum planet outside of the J - Rakuyou - Kamui's early life was not an easy one. His father, a bounty hunter, was barely home, and his mother was severely ill - and only getting worse the longer she held on - so this left Kamui to fend for his little family on his own more often than not. Once, on the rare occurrence his father was home, he took both Kamui and his little sister for modifications. He and their mother had originally come from a dying planet where it was a tradition of sorts to do, one he had no intention of letting pass by his children, despite the family being misplaced from their home planet.

And so they went.

And so his father went, again. As usual.

Kamui quickly got used to the modifications. He was faster, now, stronger, better able to defend himself and his sister and mother, better able to take care of their family, and he did. It came at a bit of a price, like everything does, but he could deal with needing to keep out of the sun, he could deal with the quick metabolism that left him always hungry, if it meant he could take care of his important people.

But it also led to him getting into fights more often. It led to him winning fights more often, but it also led to disappointment and worry. He'd often come home bruised and bloodied, would come home to a gentle lecture and kind hands patching him up and, eventually, his mother him promise not to fight anymore.

He didn't, striving to be a good son, to not be a disappointment, even as his mother grew more ill, even as his family was ridiculed - she's a plague, they're trash among trash, they should all just die. A gang of street thugs made the mistake of picking on his little sister, and he broke his promise. As luck would have it, there was a witness. A space pirate. Another descendant of Kouan. A man, he thought, who could take them away from here, since his father was failing to deliver promised aid.

There was one little problem. The man wasn't alone. He wasn't in charge of whether or not he could take along a brat and his family. So Kamui spoke to his captain. He fought his captain. And as he lost the fight, he nearly lost his life, if not for the intervention of the man. A deal was made. He could come back to them as many times as he wanted while they were still on the backwater planet of his, and if he could land a strike before they left, they would give him - and his family - room on their ship.

Eventually, he managed. He managed, and he had to try and convince his mother to leave with him, on a day he knew his father would be coming home. It didn't go well. She didn't want to leave, and Kamui and his father fought. He took his arm, and nearly lost his life in the process.

He left Rakuyou alone.

Spending the rest of his childhood among pirates led Kamui down a path of violence, but the crew - watching him grow up and integrate himself as one of them - quickly became endeared to him, so when the former captain retired, they were quick to fall in line behind him without question.

It was then, too, that they were sent as a vanguard to the J - the Seventh Division of a larger coalition of pirates, with a reputation for being especially ruthless, and clearly the best choice for exploration, and those with the best chance of returning with information and - if possible - a spread of influence.

It didn't quite go as planned.

Their ship broke down soon after arriving in the quad. They couldn't make repairs with the tech available. They dispersed, with instructions to come back together... eventually, provided everyone was still alive and committed. Simple. Easy.

And here is where his true entry to the world begins. Alone in a supposedly new place, having to make his way with false memories.

Kamui himself has spent his time in an odd state between leisurely indifference and driven commitment. He's made connections, never spending too long in one place as he's zipped around the quad, dipping his fingers into all manner of things and - perhaps surprisingly - trying not to make too many enemies, using his amicable nature and easy smile to his advantage. He's even grown his crew, used those connections to add several more miscreants to the ranks of those he believes are out there, working to - if not go home, then to at least leave the J for more familiar space.