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Affiliation: Order of the Phoenix
Sirius is Harry's godfather, murdered in Book 5, Harry Potter and the Order of the Phoenix, by Bellatrix Lestrange. Once upon a time, Sirius was in Gryffindor House where he became good a close friend of James Potter, Harry's dad. Later, Sirius fought against Death Eaters and was framed for betraying James and Lily Potter and murdering a number of other Muggles. He was sentenced to life in Azkaban Prison, but he escaped and joined up with the Order of the Phoenix.
We learn more about his murder at the beginning of Book 6 from Cornelius Fudge:
"Black? Black?" said Fudge distractedly, turning his bowler rapidly in his fingers. "Sirius Black, you mean? Merlin's beard, no. Black's dead. Turns out we were – er – mistaken about Black. He was innocent after all. And he wasn't in league with He-Who-Must-Not-Be-Named either. I mean," he added defensively, spinning the bowler hat still faster, "all the evidence pointed – we had more than fifty eyewitnesses – but anyway, as I say, he's dead. Murdered, as a matter of fact. On Ministry of Magic premises. There's going to be an inquiry, actually…" (1.67)
In Chapter 3 we learn that Sirius has left Harry everything he owns: gold, possessions, and his house, number twelve, Grimmauld Place. Harry also inherits his house elf, Kreacher, who hates Harry Potter, but who has no choice but to serve him. Harry still mourns the loss of his godfather, for Sirius was the only real family he had left.