Sirius Black. Harry's godfather, schoolfriend of James Potter

In Harry Potter and the Prisoner of Azkaban, Sirius Black has just escaped from the infamous wizard prison and is on the loose. Before his escape he was heard to whisper in his sleep 'He's at Hogwarts...'. Black was imprisoned for the murder of one Peter Pettigrew, and a street full of Muggles. When Harry's parents knew they were being sought by Voldmort, they protected themselves by use of a Fidelius charm. This spell can utterly hide any single piece of information, such as the location of Harry's family, so it cannot be learned by anyone save that they are told by the designated Secret-Keeper. The Secret-Keeper is the only person who knows the important secret. So far as anyone knew, Sirius Black was the Potters' Secret-Keeper and therefore he must have told Voldemort where they were hiding. In fact, Pettigrew was the Secret-Keeper and it was he who betrayed his friends, Harry's parents. Black went after Pettigrew, who faked his own death to escape, leaving the blame for the massacre to fall squarely on Black's shoulders. When Black learned Pettigrew was hiding in Hogwarts, in the shape of Ron Weasley's pet rat Scabbers, he contrived his escape from Azkaban and came to protect Harry.

Since the events of Harry Potter and the Prisoner of Azkaban Harry has learned that Sirius Black, and Remus Lupin, were his father's two closest friends, and together they caused mayhem at Hogwarts in their school days. Sirius is Harry's godfather and as such is responsible for Harry's welfare, though he has never been cleared of blame and remained in hiding until his fall through the veil in the Ministry of Magic's Department of Mysteries. Black is a pure blood wizard, last surviving member of one of only a very few remaining pure blood families. As such he is related to nearly all the other pure blood families, including the Malfoys and many other Death Eaters.