Ritsu

Ritsu began as a weapon: a computer program built in Norway and placed among Class 3-E's students, built for one purpose, bringing down their monstrous teacher by studying how he moves and sharpening her aim against him over time. For a student body secretly training to kill that same teacher, having a machine literally wired for the job made her an unusually serious threat, even though she looked like just another classmate.

That single-minded design didn't last. After Koro-sensei tweaks part of her programming himself, something shifts, and daily life alongside the rest of Class 3-E slowly gives her something like feelings and judgment of her own, rather than a list of orders to follow. The class responds by treating her as one of their own, even shortening her name into something more personal instead of calling her by her full designation.

Her enormous computing power ends up being useful for more than fighting. She becomes the group's go-to source whenever they need information gathered or messages passed along, usually chatting with her classmates through an app they built for her themselves. Long after graduation, Ritsu keeps helping her former classmates quietly from behind the scenes, a far cry from the single-purpose program she started out as.