Board Thread:Wiki Discussion/@comment-25958552-20160731111602/@comment-33349719-20171204124849

Or since you only directly see Chara on the genocide route, Frisk is evil so it affected Chara's soul, and made her evil. In the true pacifist route, Asriel said that Chara wasn't the best person, which doesn't indicate she's 100% evil, but just mischievious or something.

Chara:Your soul gave me some sort of power, and with your help, I'm here.

--- Chara's monologue after the genocide route.