William

“Albus Severus," Harry said quietly, so that nobody but Ginny could hear, and she was tactful enough to pretend to be waving to Rose, who was now on the train, "you were named for two headmasters of Hogwarts. One of them was a Slytherin and he was probably the bravest man I ever knew.”
― J.K. Rowling, Harry Potter and the Deathly Hallows