Severus Snape is the Potions Master at Hogwarts and head of Slytherin House. He is a tall man with greasy black, shoulder-length hair and a hooked nose. He has an unusually unpleasant personality. However, he is an expert potions brewer and he holds a position of some authority and respect under Dumbledore. Just what event in Snape's past has earned him this level of respect is unknown, but it is clear that the Headmaster has given Snape a second chance, as he has done with so many others.

Snape teaches Potions classes. His teaching style is based on intimidation and bullying. He favors students of his own house while actively undermining the efforts of students from Gryffindor. He is particularly antagonistic toward Harry, based partly on built up grudges against Harry's dad. He is also extremely cruel to Neville Longbottom for no discernable reason except that he can sense that Neville is afraid of him. Snape is a vicious, nasty man whose behavior is often inexcusable, such as when he sees Hermione with her teeth expanded past her chin and says "I see no difference."

Snape attended Hogwarts in the 1970s along with James Potter, Sirius Black, and their friends. He was already then deeply interested in the Dark Arts, and came to Hogwarts knowing more curses than most upperclassmen. His unpleasant personality was also in evidence, and he spent a lot of time following James and the others around, trying to find ways to get them expelled. When Snape became too curious about where Lupin went every month, Sirius tricked him into entering the passage under the Whomping Willow to find out. He would have been killed by the werewolf if James Potter hadn't realized what was happening and pulled him back. Snape has held a grudge about this incident for years and when he got the chance to catch Sirius and turn him over to the Dementors, he tried to do so, turning a deaf ear to the truth that Sirius was innocent. It was at this point that the full extent of his paranoia and mania became evident. Harry, Ron, and Hermione were forced to stun him to keep him from essentially murdering Sirius.

Snape was part of a group of Slytherin students in the 70s who all became Death Eaters. He served Voldemort for a time, but a some point turned against the Dark Lord and became a spy for Dumbledore. Snape still has the Dark Mark on his arm. When Voldemort returned to power in 1995, Snape agreed to perform some task for Dumbledore, although the details of this task are not known. Before setting out on this task, he met and shook hands with Sirius Black. While this was not a handshake of friendship, it was at least an acknowledgement that the two of them were now fighting on the same side against Voldemort.

Snape, Severus
hair:
black and greasy, shoulder length
eyes: 
beetle-black but without the friendly sparkle of Hagrid's
teeth: 
uneven, yellowish (PA14) [thanks Heather!]
complexion: 
sallow
distinguishing features: 
long nose, scowl
age:
35 or 36 (born c.1960) (CR)
House: 
Slytherin
robes:
billowing, black
field: 
potions
office:
located adjacent to the dungeon classroom where he teaches potions