The greasy haired, hook nosed potions master in Harry Potter that "loathes"...mostly everything. Mysterious, and has a haunting past. Was a death eater, but turned spy for Dumbledore and the Order Of the Pheonix.
In the sixth book, Snape murders Dumbledore.
In the seventh book, Snape shows Harry why he did it. Turns out he loved Harry's mother, Lily, and turned to Dumbledore for help when he found out Voldemort was going to kill her (and her son and husband). When Dumbledore has one year left to live he asks Snape to kill him in order to help others.
Dumbledore: But this is touching Severus, have you grown to care for the boy, after all?
Severus Snape: For him? Expecto Patronum!
(A silver doe patronus like Lily Potter's emits from Snape's wand.)
Dumbledore: After all this time?
Lily Potter: Does it make a difference, being Muggleborn?
Snape hesistated. His black eyes, eager in the greenish gloom moved over the pale face, the dark red hair.
Snape: No, It doesn't make any difference.