Attened Hogwarts for 7 years, was part of the infamous group of pranksters' known as the 'Marauders', Remus Lupin(Moony), Peter Pettigrew (wormtail), himself, and James Potter (Prongs). He was framed by Peter Pettigrew after the deaths of Lily and James Potter, and sent to Azkaban prision for 12 years before he escaped! Something one had ever done before.
Sirius is also an animagi, he transforms into a large, black, bear-like dog. Known as Padfoot, and/or Snuffles.
He was a fan-damn-tastic (straight) hottie.
Dumbledore: "Sirius was a brave, clever, and energetic man, and such men are not usually content to sit at home in hiding while they believe others to be in danger."