Who is Professor Severus Snape from Harry Potter and the Deathly Hallows and what is their importance?

Professor Snape is the headmaster at Hogwarts. He became a Death Eater before Voldemort's first fall from power, but because of his great love for Harry's mother, he traded his allegiance and spent his life trying to protect Harry. Snape remains in Voldemort's good graces because Dumbledore's insists that he continue to pretend to be a Death Eater. In reality, Snape secretly fights against Voldemort. In the end, Snape's actions can be understood, but up until that point, he is regarded by Harry's supporters as the enemy. It must be this way if he is to have inside knowledge of Voldemort's actions.