Sirius Black was one of the most loyal characters in the Harry Potter universe, who unfairly spent 12 years in Azkaban for a crime he didn’t commit, and his only mistake was giving up being a Secret Keeper. Throughout the book and film series, audiences met several characters from the Wizarding World, some evil (like Lord Voldemort) and others more on the side of the light (like Hermione Granger and Ron Weasley). Others had some truly tragic stories, full of misunderstandings, injustice, and loneliness. Such was the case of Sirius Black, Harry’s godfather (and father figure), whose only “crime” was to give up the role of Secret Keeper to protect his best friends.
A Secret Keeper is a witch or wizard designated to hide a secret through the Fidelius Charm, a very difficult and potent charm that conceals a secret inside an individual’s soul. The only way for others to learn the secret is for the Keeper to voluntarily and directly reveal it – methods of coercion such as the Cruciatus Curse, the Imperius Curse, Legilimency, or Veritaserum have no effect on the Fidelius Charm. If a location is hidden through this method, it can’t be revealed even if another person is staring at it, which is why Albus Dumbledore suggested it to the Potters in order to keep them (and Harry) safe.
Sirius Black, James Potter’s best friend, offered himself to be their Secret Keeper but changed his mind and suggested Peter Pettigrew instead. The reason for this was that Sirius came to distrust their friend, Remus Lupin, thinking he might have been a spy, thus making him a target of Lord Voldemort. In order to divert attention, he suggested a less likely target: Peter Pettigrew. Sirius kept this information away from Remus, and Pettigrew became the Potters’ Secret Keeper.
This turned out to be a huge mistake, though, as Pettigrew was the actual spy, and informed Lord Voldemort of the Potters’ location, resulting in their murders. Sirius was then tricked by Pettigrew and sent to Azkaban for the murder of Pettigrew and 12 muggles in front of over 50 witnesses, when in reality it was Pettigrew who killed them and later fled the scene in his Animagus form (a rat), leaving behind his severed index finger.
Sirius Black was considered one of the biggest traitors in the Wizarding World for years, before the truth about Pettigrew was revealed in Harry Potter and the Prisoner of Azkaban. Meanwhile, Pettigrew was regarded as a victim, of whom “all that was left, (was) a finger”. It was all eventually cleared and Sirius was finally recognized as the loyal friend he always was, and Pettigrew as the rat (literally and figuratively) he had always been.