Severus Snape

Severus Snape was a teacher at Hogwarts. For many years, he was the Potions teacher; however, he applied yearly for the Defence Against the Dark Arts job, which he was finally given in Harry Potter and the Half-Blood Prince.

Snape has always hated Harry Potter, because of a lasting grudge against Harry's father, James Potter, who attended Hogwarts at the same time as Snape. James bullied Snape while they were at school. However, on one occasion he also saved Snape from visiting the Shrieking Shack, where he would have been attacked by Remus Lupin in werewolf form, thereby probably saving Snape's life.

Snape's mother was a witch named Eileen Prince and his father, Tobias Snape, was a Muggle.

While at Hogwarts, Snape took potions classes with Lily Evans, who became Harry's mother and was described by the then potions master, Horace Slughorn, as his best student. Harry is lent an old potions textbook for his sixth year classes which contains handwritten tips about how to make better potions, and a few spells written by Snape in or before his 5th year at Hogwarts. The book is inscribed 'property of the Half-Blood Prince', and at the end of book six Snape tells Harry that the nickname referred to him. However, he did not use the name at the school; Harry asked Remus if he knew the name, but he did not.

Horace Slughorn actually said that Harry performed a potion that Lily had performed too, but Snape had failed. As Harry was using Snape's book, we can guess Snape gave his secrets to Lily instead of using them. It can support the theory "Snape loved Lily" or meaning Snape wanted to be underestimate.

Snape became Potions Master, but he was particularly talented in Dark Magic. He was described as knowing more curses when he arrived at the school than most knew when leaving it. He invented several spells, including:
 * Levicorpus (which grabs the victim by the ankle and dangles them upside-down)
 * Muffliato (which fills the ears of anyone nearby with an "unidentifiable buzzing sound")
 * Langlock (which glues the victim's tongue to the roof of his or her mouth)
 * Sectumsempra (which slashes the victim continuously).

Snape eventually became a Death Eater, and was responsible for telling Lord Voldemort about the prophecy of the one who would kill the Dark Lord. He overheard Sybill Trelawney telling the prophecy to Albus Dumbledore at the Hog's Head Inn. At that time it was not clear who was meant by the prophecy. Afterwards it became clear it referred to either Harry or Neville Longbottom, and led to Voldemort seeking the deaths of James, Harry, and Lily. Snape revealed to Dumbledore that he had been the one to overhear the prophecy, and became a member of the Order of the Phoenix, accepted largely because Dumbledore vouched for him. Why he defected, and whether it was his admission alone that so convinced Dumbledore, is unknown.

Interesstingly, we kwow that Dumbledore had spies in the Death Eaters, and that one of them warned Dumbledore that the Potters were the next target. It would be interessting to kwow if Snape was this one. Nothing is sure as Flickwick spoke of spies on the plural.

In Harry Potter and the Half-Blood Prince, Snape traitorously leads the Death Eaters in the Battle of Hogwarts where he eventaully kills Albus Dumbledore using the unforgivable curse Avada Kedavra, although his next action is to order the Death Eaters to leave Hogwarts without harming Harry, and then to give Harry a vital lesson in serious duelling. It seems likely Snape was compelled to kill Dumbledore by an unbreakable magical vow he swore at the start of Half-Blood Prince to Draco's mother, with whom he had restablished contact once the Death Eaters resurfaced. Exactly what the Vow concerned remains unclear, but he undertook to protect Draco Malfoy and ultimately carry out whatever task Draco was set by the Dark Lord should Draco fail, suggesting the boy had been tasked with murdering Dumbledore.

Snape's career as a double agent, whichever side he was really working for, is over: while he must surely now be considered a loyal Death Eater by Voldemort and his followers, his credibility and contact with the Order of the Phoenix is lost, and he cannot pass information between the groups any longer.