Professor Severus Snape
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While Professor Snape (the Hogwarts Potions Master and Head of Slytherin House) is a huge character in the Harry Potter series as a whole, he plays a relatively minor part in Harry Potter and the Chamber of Secrets. Professor Snape hates Harry's guts, so when he sees Harry and Ron arriving at Hogwarts in their flying car, he threatens them with expulsion ["So […] the train isn't good enough for the fames Harry Potter and his faithful sidekick Weasley. Wanted to arrive with a bang, did we, boys?" (5.131)]. Professor Dumbledore and Professor McGonagall step in and stop him, to Snape's disappointment.
Professor Snape also assists Professor Lockhart in running the Dueling Club, seizing the opportunity to blast Professor Lockhart against a wall. Professor Snape arranges for Draco and Harry to fight each other, leading to Draco conjuring the snake that shows the whole school Harry's a Parseltongue. Professor Snape is also among the professors who encourage Professor Lockhart to go to the Chamber of Secrets after Ginny Weasley just so that they can get Professor Lockhart out of the way as they get down to business. If you want to see more of Professor Snape, try Book 3, 5, or 7...