Severus Snape
Snape is the Potions master at Hogwarts school of Witchcraft and Wizardry. He is a bitter man and hates Harry with a passion all because James Potter picked on him while they were in school. Actually, James was just plain mean to Snape (but my opinion doesn't matter.) Snape had a fairly rough childhood (again in my opinion) due to (this is from the fifth book) when Harry broke into Snapes mind in their Occlumency lessons. Rowling showed Snape in his room alone, his parents (or so I think) fighting, and him crying in a corner...if you put those together it really makes you think. He saved Harry in Harry's first year from Quirrel's dark charm because while Snape was in school James saved his life when Sirius was stupid and showed him to Lupin when he was in his werewolf state. James got a hold of this information and prevented this from happening...all in all holding a life debt between the two. Snape was a death eater and ultimately switched sides at the end (no one knows why) and Dumbledore still trusts him no matter what the others think. He still makes sure Harry is miserable during Potions class and probably always will. He got the defense job finally. Then proceeded to end the year with a bang.... he killed Dumbledore after making a pact with Narcissa (the unbreakable vow) that he would help Malfoy finish the task. He was also the Half-Blood Prince.