Sirius Black, nicknamed Padfoot, is the last heir to the House of Black, a once notable pure-blood Wizarding family. He is the brother of Regulus Arcturus Black and a cousin of Bellatrix Lestrange. He rejected his family's pure-blood elitism and reverence for the Dark Arts. In contrast to his home life, Sirius greatly enjoyed life at Hogwarts. He was proud of the fact that he was the only Black to be sorted into Gryffindor. He was inseparable from his best friend James 'Prongs' Potter and befriended Remus 'Moony' Lupin and Peter 'Wormtail' Pettigrew.
Sirius, along with James, bullied Snape, nicknaming him "Snivellus", from his first name, Severus, while they were at school. Sirius left home at the age of sixteen and took refuge with James and his parents. His outraged mother burned his name off the family tree, but he was left financially independent by his Uncle Alphard's generous bequest (hence Alphard's name was burnt off as well). After leaving school, he remained close friends with James and became close to his wife, Lily, after being best man at their wedding. When Harry was born, the Potters named Sirius as his godfather.
When the Potters went into hiding from Voldemort, Sirius was going to be their Secret-Keeper. Sirius, however, convinced James and Lily to switch Secret-Keepers at the last moment to Peter Pettigrew, believing that Voldemort would never think to hunt him and would attack Sirius instead. However, Pettigrew betrayed James and Lily, and they were murdered by Voldemort.
Harry survived and was rescued by Rubeus Hagrid, the gamekeeper of Hogwarts. Sirius asked Hagrid to give Harry to him, since he was Harry's godfather, but Hagrid, under Dumbledore's orders, refused and instead took Harry to live with the Dursleys. Sirius gave Hagrid his flying motorbike, saying he wouldn't need it. In his rage, Sirius tracked Pettigrew down, but during the confrontation Pettigrew faked his own death and killed twelve Muggles, framing Sirius for betraying the Potters and murdering Pettigrew and the Muggles. Sirius was arrested and sent to Azkaban without trial by Bartemius Crouch Senior. Unlike most of the other Azkaban prisoners, Sirius was able to keep his sanity because he knew he was innocent.
Twelve years later in Harry Potter and the Prisoner of Azkaban, Sirius sees a picture of the Weasley family on the front cover of the Daily Prophet, in which a rat stands perched on Ron's shoulder. The rat is Ron's pet, Scabbers, and Sirius immediately recognises Pettigrew in his Animagus form, seeing a toe was missing. This knowledge clears his mind and enables him to escape Azkaban, accomplishing this feat by transforming into his Animagus dog form. His severe weight loss from malnutrition and the Dementors' lack of ability to differentiate between unsophisticated canine emotions and insanity allows him to slip through his cell bars. He then swims to shore. After his escape, Sirius takes refuge in and around Hogsmeade. Knowing that the Dementors are stationed around Hogsmeade and at Hogwarts, Sirius remains in his Animagus form during this time, and is able to enter the Hogwarts grounds without being detected by the Dementors. He makes several unsuccessful attempts to kill Pettigrew, until his presence is detected by the students and teachers of Hogwarts when he tries to enter Gryffindor Tower, and the school's security is increased. Towards the end of Prisoner of Azkaban, Harry, Ron, and Hermione confront Sirius and Harry attempts to kill him. Lupin, however, who has learned that Sirius is innocent after seeing Pettigrew's name on the Marauder's Map he had previously confiscated from Harry, prevents Harry from doing so.