Someone who often competes with the protagonist and they motivate each other to improve themselves. The grim, volatile foe of the protagonist, who, through a series of inexplicable events up to and including attempted murder, gains grudging respect for said hero/heroine (and may even save them) going from intolerable ass to tolerable ass. Frequently a Foil for the hero, who's ultimately a Mirror Character. Often they show what the hero could have become if fate was just a little bit different.