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Sirius Black is a fictional character in J. K. Rowling's Harry Potter series, played by Gary Oldman. Black, also known as Padfoot or Snuffles (in his Animagus form) was a pure-blood wizard. He was born on 3rd November, 1959 and is the eldest son of Orion and Walburga Black. He is heir of the Ancient and Noble house of Black and brother to Regulus. His cousin's are Bellatrix Lestrange (nee Black), Andromeda Tonks (nee Black) and Narcissa Malfoy (nee Black). He was Harry Potter's godfather. He was best friends with James Potter, along with Remus Lupin and Peter Pettigrew. These four were known as the marauders in their school days, and created the Marauder's Map.
He, along with his three friends, belonged to Gryffindor house when at Hogwarts, unlike the rest of his family, who had belonged to the house of Slytherin. He did not like his family, and ran away from home to James Potter's house when he was 16. The Potters took Sirius in as their own son. His prized possession during childhood was his flying motorbike.
Sirius was an animagus and was able to transform into a big black dog, (a grim) which is an omen of death, at will. He, along with James Potter and Peter Pettigrew became animagus to help Remus Lupin with his transformations. He was the first wizard to escape Azkaban prison, after being kept there twelve years for a crime that he did not commit. The betrayal of James and Lily Potter, the murder of Peter Pettigrew and 12 muggles with an unknown curse. Harry found out, after a year of believing otherwise, that Sirius was an innocent man, and helped him evade capture from the ministry of magic. Sirius went on the run before returning to his (empty) childhood home, 12 Grimmauld Place, and offered it up as headquarters for the Order of the Phoenix upon Voldemort's return. He was killed in a battle at the Ministry of Magic by his cousin, a Death Eater, Bellatrix Lestrange in Harry's ( his Godson's )5th year at Hogwarts. He returned during Harry's 7th year along with James, Lilly and Remus, after being resurrected with the help of the Resurrection stone.
He was the only remaining “true” heir of the House of Black. 12 Grimmauld Place was gifted to Harry Potter, Sirius’ godson. In which case, Harry or Draco Malfoy (related to the House of Black through his mother Narcissa) would’ve likely inherited the title of Head of House. Possibly even Andromeda Tonks or her grandson, Edward “Teddy” Remus Lupin.
As he has said before, "almost all pure-bloods are somehow related, and he’s no exception". He’s related to the Weasleys by way of Cedrella Weasley (nee Black), whom married Septimus Weasley. Together they're the paternal grandparents of the Weasley children. He’s related to the Malfoys through Narcissa Malfoy (nee Black) who married into the Malfoy family, and is Sirius’ cousin. He’s even related to his godson, Harry Potter, through Dorea Potter (nee Black) who is a distant relative to Harry and James Potter. It is said, some people think, that Sirius's Boggart is a dead Harry Potter.