Quirenius Quirrell
Yeah, bad man. He tried to kill Harry in Harry's first year. He always wore a turban around his head that reeked of onions and he was a paraniod guy that always stuttered. But all of this was basically a cover in someways literally. Because apparently he had what was left of Voldemort stored inside of him. He had to drink unicorns blood to keep both of them alive and in the end...Quirrell was killed. Thus the end of his career in defense against the dark arts.