Severus Snape
Potions master and leader of Slytherin House at Hogwarts school, Snape is loathed by all students except his house followers. Snape went to Hogwarts quite a few years after Dumbeldore - and competed in almost everything, from high marks to position of Head Boy, with Harry's father - James Potter. But the worst thing about his grudge against James Potter is the fact that he saved Snape's life - a fact which makes him crawl at the skin thinking he will forever be in debt to his enemy. 

Grossly unfair to anyone not in Slytherin, Snape is bad-tempered, mean, rude and conspiracing - but he plays a big part in saving the school whenever it gets in trouble. Unfairly thought of as Dark Wizard, Snape has in fact saved Harry's life on one occassion. When finally the debt was repaid to his late enemy, James Potter, Snape went back to his bad-tempered self - giving detentions left and right, blaming Harry and his friends for almost everything, and unfairly favouring Slytherins Draco Malfoy and his friends.
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