Okay, so 'Fort of Apocalypse' – I think that's the manga, right? The one with the prison island setting when the world ends. Main guy is definitely Maru, the quiet type who gets thrown in there before everything goes to hell. Then you've got his cellmate, Joe, who's more hot-headed and ends up being the brawn to Maru's brain, sort of. There's also a warden's daughter, I wanna say her name is Maria? She gets trapped inside with them after the outbreak hits.
What's interesting is it starts with a pretty big cast of inmates and guards, but they get whittled down fast. The key dynamic is really Maru trying to keep a small group alive, balancing survival with holding onto some shred of humanity. There's this one guard character, a real hardcase, who becomes a major obstacle – turns into almost a secondary villain within the prison walls. The characters aren't super deep archetypes, but the pressure-cooker situation makes their alliances and betrayals matter more than backstories.