Sirius Black III was a pure-blood wizard, the elder son of Orion and Walburga Black, and the godfather of Harry Potter. Rejecting his family's belief in pure-blood supremacy, he was sorted into Gryffindor House at Hogwarts School of Witchcraft and Wizardry. His closest friends were James Potter, Remus Lupin, and Peter Pettigrew, and together they formed the group known as the Marauders. An unregistered Animagus, Sirius could transform into a large black dog known by the nickname Padfoot. A member of the original Order of the Phoenix, Sirius was framed for the betrayal of James and Lily Potter and the murder of Peter Pettigrew and twelve Muggles. He was sentenced to a life term in Azkaban without a trial. After twelve years, he became the first person known to escape the prison unassisted. He proved his innocence to Harry and his friends and spent the remainder of his life as a fugitive, eventually rejoining the Order of the Phoenix. He was a crucial paternal figure for Harry until his tragic death at the hands of his cousin, Bellatrix Lestrange, during the Battle of the Department of Mysteries.
Born into the ancient and wealthy Black family, Sirius grew up at Number Twelve, Grimmauld Place. From a young age, he disagreed profoundly with his family's obsession with blood purity and their affinity for the Dark Arts. This defiance led to his sorting into Gryffindor, breaking a long-standing family tradition of being sorted into Slytherin. At sixteen, he ran away from home and was taken in by James Potter's family. As a result, his enraged mother, Walburga, blasted his name off the Black family tree tapestry. He had a younger brother, Regulus Arcturus Black, who initially followed the family's path and became a Death Eater before turning against Lord Voldemort.
At Hogwarts, Sirius was popular, clever, and handsome, but also known for his arrogance and bullying of students he disliked, particularly Severus Snape. He formed an inseparable bond with James Potter, Remus Lupin, and Peter Pettigrew. Upon discovering that Lupin was a werewolf, Sirius, James, and Peter secretly became Animagi in their fifth year to provide Lupin with companionship during his transformations. Sirius's form was a large black dog, earning him the nickname Padfoot. The four friends used their knowledge of the castle's secret passages, aided by their transformations, to create the Marauder's Map.
After graduating from Hogwarts, Sirius and his friends joined the Order of the Phoenix to fight against Lord Voldemort. He was the best man at James and Lily's wedding and was named Harry's godfather. When the Potters went into hiding under the Fidelius Charm, Sirius was their initial choice for Secret-Keeper. However, fearing he was too obvious a target, he persuaded them to switch to Peter Pettigrew, a decision that would prove fatal. Following Pettigrew's betrayal and the Potters' murders, a grief-stricken Sirius tracked Pettigrew down. Pettigrew created a massive explosion that killed twelve Muggles, cut off his own finger, and transformed into his rat form, framing Sirius for the entire crime. Sirius was arrested by the Department of Magical Law Enforcement and imprisoned in Azkaban without a trial, as Bartemius Crouch Sr. believed the evidence against him was overwhelming.
Sirius spent twelve years in Azkaban, surrounded by Dementors. He maintained his sanity by focusing on his innocence and by transforming into his dog form, as the Dementors' emotional perception was less acute towards animals. In 1993, he saw a picture of the Weasley family in a newspaper, where he recognized Pettigrew in his rat form (as Scabbers) on Ron Weasley's shoulder. This knowledge gave him the strength to escape, slipping past the Dementors in his emaciated dog form.
Sirius made his way to Hogwarts to protect Harry from Pettigrew. His gaunt, frightening appearance led to a school-wide panic. The truth was finally revealed in the Shrieking Shack, where Sirius and Lupin confronted Pettigrew. With the help of Harry, Ron, and Hermione Granger, they exposed Pettigrew, but he escaped before his guilt could be proven to the Ministry of Magic. Sentenced to the Dementor's Kiss, Sirius was saved by Harry and Hermione using a Time-Turner and escaped from Hogwarts on the back of Buckbeak the hippogriff. For the next two years, he remained a fugitive, communicating with Harry through letters and the Floo Network.
Following Lord Voldemort's return, Albus Dumbledore reinstated the Order of the Phoenix, and Sirius offered his childhood home, 12 Grimmauld Place, as its headquarters. Confined to the house due to his fugitive status, Sirius grew frustrated and reckless. His relationship with the family house-elf, Kreacher, who was loyal to Sirius's mother, was one of mutual contempt. In 1996, Lord Voldemort exploited Harry's connection to him by planting a false vision of Sirius being tortured in the Department of Mysteries. Though Snape tried to verify Sirius's safety, Kreacher lied to Harry, confirming his master was not at home. Harry and his friends rushed to the Ministry of Magic, falling into a trap. Members of the Order, including Sirius, arrived to rescue them. During the ensuing Battle of the Department of Mysteries, Sirius duelled his cousin, Bellatrix Lestrange. While taunting her, he was struck by a spell (an unspecified red jet of light) that sent him staggering backward through a mysterious, veiled archway, leading to his death.
In his youth, Sirius was described as tall and handsome with long, black hair and striking grey eyes. His twelve-year imprisonment in Azkaban left him with a gaunt, skeletal face, waxy skin, and yellowed teeth. After his escape, with proper care, his handsome looks began to return, though he retained a “shadow of prison” in his eyes. Sirius was fiercely loyal, brave, and deeply affectionate towards those he loved, particularly James and Harry. He acted as a loving and protective godfather. However, his years in Azkaban stunted his emotional maturity, leaving him bitter, reckless, and prone to volatile moods. He could be arrogant and cruel, as shown by his lifelong animosity towards Severus Snape and his mistreatment of Kreacher, which ultimately contributed to his own demise. He often treated Harry more like a peer or a substitute for James rather than a godson, encouraging him to take unnecessary risks.