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House: Head of Slytherin
Affiliation: Death Eater?
Professor Snape is the head of Slytherin House; he's also the Hogwarts Potions instructor. He's Harry's main adult antagonist at school. Professor Snape remains a mystery to us throughout Goblet of Fire (and in fact, for much of the series). The thing is, everything about him screams "evil!", but Professor Dumbledore trusts him.
On the evil side, Professor Snape is filled with the most profound loathing and disgust for our hero, Harry Potter, and he has acted that way since Harry was only eleven years old. When Snape catches Harry, Ron, and Hermione talking about Rita Skeeter's latest Daily Prophet article in Potions class, he whispers to Harry,
You might be laboring under the delusion that the entire wizarding world is impressed with you [...] but I don't care how many times your picture appears in the papers. To me, Potter, you are nothing but a nasty little boy who considers the rules to be beneath him. (27.46)
And Snape goes further, threatening to slip a powerful truth serum into Harry's pumpkin juice when he's not looking so that he can prove that Harry has been breaking into his office. Pretty bad, huh?
The evidence for Professor Snape's badness mounts throughout Goblet of Fire. He clearly has some kind of history with Professor Karkaroff, whom Sirius knows was a former Death Eater. In Professor Dumbledore's Pensieve memories, Harry finds out that Snape only escaped prosecution for being a Death Eater thirteen years prior because Dumbledore vouched for him. Dumbledore won't explain to Harry why he trusts Snape, saying only, "[it is] a matter between Professor Snape and myself" (30.202). However, Mad-Eye Moody certainly doesn't trust Professor Snape further than he can throw him, threatening:
Course Dumbledore trusts you [...] He's a trusting man, isn't he? Believes in second chances. But me – I say there are spots that don't come off, Snape. Spots that never come off, d'you know what I mean? (25.111)
The "spot" that Moody is referring to with so much emphasis is clearly the Dark Mark on Professor Snape's left arm, which proves that he was once a Death Eater. We actually get to see the Mark in Chapter 36, when Snape furiously shows it to Cornelius Fudge to prove:
This Mark has been growing clearer all year. Karkaroff's too. Why do you think Karkaroff fled tonight? We both felt the Mark burn. We both knew he had returned. (36.153)
So Professor Snape is willing to expose his Dark Mark to Cornelius Fudge in support of Professor Dumbledore. There's something between Snape and Dumbledore that we are still just missing. We know that Professor Snape was once a Death Eater. We also told that Snape is good – Professor Dumbledore says that he and Sirius "are on the same side now" (36.179), after all. But we still have to find out two major things about Professor Snape: why does Professor Dumbledore trust him so much? And what is this mysterious mission that Dumbledore "must ask [Snape] to do" (36.187), even though it's going to put Professor Snape in great danger? Obviously, we have to keep reading the series to find out.