[quote=@RadioactiveRat]
Two bits come to mind:
I am reminded of deadman wonderland, which would be fun to play in this prison scenario.
I want to either create my own faction or remain independent but I can't make up my mind because Manuel is a violent and short-tempered asshole who is the most well known arena fighter in the prison. If someone could stay independent it would be him,  but I don't want to make a typical mary-sue. Mamaging a faction is too much for Manuel, and he wouldn't be anybody's bitch as a part of a faction.

Could you help me out here?
[/quote]

Technically that would be a Gary-Sue, not that it's important.

Generally the factionless that survive do so because there smart and not impulsive. You need to be able to draw as little attention to yourself as possible but if it comes down to it also be able to defend yourself. Manuel doesn't really strike me as the type of person that is capable of flying under the radar. So that might not be a good fit for you.

Since Manuel is technically immortal you could make it so that if he is ever severly injured or killed the guards are tasked with dragging him into the arena or the auction house so that he can recover/come back to life. Which is one way you could remain independent but not invulnerable.

On the other hand he could be a member of a faction but sort of more of a fringe member. As a champion arena fighter he'd have some influence in the prison so a faction could claim him looking to exploit that influence while he could use the faction name to protect himself but not actually have to involve himself in faction business very often.