Severus Snape is a fictional character in the Harry Potter book series written by J.K. Rowling. In the first novel, Harry Potter and the Philosopher's Stone, he is one of the primary antagonist. As the series progresses, Snape's character becomes more layered and complex, and Rowling does not fully reveal the details of his true loyalties until the end of the final book, Harry Potter and the Deathly Hallows. Snape appears in all seven novels of the series.
Over the course of the series, Snape's portrayal evolves from that of a malicious and partisan teacher, to that of a pivotal character of considerable complexity and moral ambiguity. Snape teaches Potions at Hogwarts, and later becomes the Defence Against the Dark Arts teacher in the sixth novel, and becomes the Headmaster of Hogwarts in the final novel. Rowling has described him as "a gift of a character".
In an interview and described Snape as a horrible teacher: "the worst, shabbiest thing you can do as a teacher is to bully students." However, she does suggest in the books that he is generally an effective teacher. It is believed that Snape is based on John Nettleship, who taught Rowling chemistry at the Wyedean School near Chepstow. For Snape's surname, Rowling borrowed the name from the village of Snape, Suffolk.
In a 1999 interview, and again in 2004, Rowling singled out Snape as one of her favourite characters to write.
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Official Site: J.K. Rowling
Links: Wikipedia Entry Severus Snape's Slytherin Society Official WB Harry Potter site