About:

Professor Severus Snape(9 January, 1960 to 2 May, 1998)was an English half-blood wizard serving as Potions Master (1981-1996), Defence Against the Dark Arts professor (1996-1997), and Headmaster (1997-1998) of the Hogwarts School of Witchcraft and Wizardry as well as a member of the Order of the Phoenix and a Death Eater. His double life played an extremely important role in both of the Wizarding Wars against Voldemort. The only child of Muggle Tobias Snape and Gobstones witch Eileen Snape (née Prince), Severus was raised in the Muggle dwelling of Spinner's End, which was in close proximity to the home of the Evans family, though in a poorer area. He met Lily and Petunia Evans when he was nine and fell deeply in love with Lily, becoming a close friend of hers.
Severus started at Hogwarts with Lily in 1971, where he was Sorted into Slytherin House. This put him in the same year as Lily but unfortunately for him in rival houses. Severus became the immediate enemy of James Potter and Sirius Black and was a frequent victim of their bullying. This led him to be irritable towards James's son Harry when he was a professor. Snape, when young, developed a passion for the Dark Arts, which increased as his desire for revenge grew stronger. Snape became involved with the bullies in Slytherin House, many of whom were pure-blood supremacists. This put his friendship with Lily, a Muggle-born, under great strain until it was eventually broken in their fifth year. In an attempt to win back Lily's affections, Snape joined the Death Eaters along with a group of his fellow Slytherins.
Shortly before Lily Evans was murdered by Lord Voldemort, Snape changed sides and became a member of the Order of the Phoenix as well as a double agent during the Second Wizarding War. With tremendous difficulty, Snape prevented Lord Voldemort from learning the truth about his loyalties. Despite the opinions of most others including Harry during his early life, Albus Dumbledore trusted Snape for reasons that were kept between them both until their deaths. Despite Snape killing Dumbledore, it is learned that they had a special agreement for him to do so. When he died, it was revealed that his deep strong love for Lily Evans caused him to redeem himself, joining Dumbledore's cause for her protection (and, after her death, that of her son) from Lord Voldemort.