Sirius Black is a fictional character in J. K. Rowling's Harry Potter series, played by Gary Oldman. He is Harry Potter's godfather. He was the best friend of Harry's father, James Potter, along with Remus Lupin and Peter Pettigrew. These four were known as the marauders in their school days, and were attributed with creating 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 disliked his family very much, and ran away from home to James' house when he was sixteen. His prized possession in his youth was a flying motorbike.
Sirius was an animagus and was able to transform into a big black dog at will. This is how he escaped from the wizard prison, Azkaban after being detained there twelve years for a crime that he did not commit (the murder of Peter Pettigrew and thirteen muggles). Harry found out, after a year of believing otherwise, that Sirius was an innocent man, and helped him evade capture from the wizarding government. Sirius went on the run before returning to his (empty) childhood home and offered it up as headquarters for the Order of the Pheonix upon Voldemort's return. He was killed in a battle at the Ministry of Magic by his cousin, a Death Eater, Bellatrix Lestrange.