Professor Severus Snape
Severus Snape is the Potions teacher at Hogwarts School of Witchcraft and Wizardry. He is also the Head of Slytherin House, and briefly served as Headmaster in book 7. Although Snape is a former Death Eater, Professor Dumbledore places his utmost faith in Snape's loyalty, though exactly why he trusts him so implicitly is initially unknown. Snape joins the Order of the Phoenix to combat Lord Voldemort, apparently acting as a double agent, though, later, Snape also serves Lord Voldemort. Snape's true loyalties remain uncertain until the final book.