In terms of bad guys, Revenent is right up there with Heartless on my roster of characters. He doesn't succeed in securing the top spot because he can actually sympathize and care about others - he just doesn't most of the time.
Formally known as 'Northern Star', Revenant basically deserted the REIN after he became disillusioned with the competency of Equestrian leadership (along with some personal trauma that he blamed on incompetence). He's joined up with WRAITH's mission to dismantle and disestablish said leadership, because that's what all the bad guys do!
...At least the cool ones. *emo hair flip*