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Snape. It rhymes with grape, tape, scrape, and drape. And it sounds a bit like "snake." Professor Snape teaches Potions at Hogwarts. He doesn't like Harry, and Harry doesn't like him. Snape is super mean to Harry and the other Gryffindors, and looooves the Slytherin students over all others. If we think Slytherin students as having a greater potential for evil than the others, then their Head of House may just have a nasty side as well. Snape is very different from the Gryffindor Head of House, Professor McGonagall; he's strict in a crueler way than she is. And, he's not very fair.
We learn from Dumbledore that Snape has a reason for disliking Harry – it's hereditary. Harry's father saved Snape's life once, and Snape never forgave him for doing so. So, now Snape is taking it out on Harry by making Potions class a lot harder than it needs to be, among other things. Snape is hard on all the Gryffindors, true, but he's especially tough on Harry and his friends.
It's pretty surprising, then, that Snape helps to save Harry's life and not end it. He performs charms to save Harry, not harm him, at the Quidditch game. Snape even steps up as a ref in an attempt to protect Harry. He also goes up against Quirrell to protect Harry, not to help Voldemort, in a complete about-face from Harry's suspicions of him.
Yet there is something dark about Snape. There may not be any evidence to support Harry's suspicions, but sometimes you just have to go with your gut.