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Severus went through a creature inheritance in his second year and ever since he has been living with his grandfathers on his mum's side.

One time Lily had enough of being Severus friend and hit him in the great hall and his grandfathers came to Hogwarts after he sent them a letter.

The time Lily hit Severus. James Potter, Sirius Black and Remus Lupin were pissed because she hit their mate. Sirius is a Vampire, James is a Veela and Remus is a Werewolf.