Sirius Black

Mister Sirius Black, Senior Marauder , is a wizard and animagus living at Hogwarts as of 1993.

Past
Sirius Black's past up until 1992 is identical to his canon self's.

1992-1993
In 1992, a visitor to Azkaban was carrying an issue of the Daily Prophet containing a message by Ron Weasley, who was looking for his pet rat Scabbers who had vanished. From the description, Black recognized the Animagus form of Peter Pettigrew, his former friend whose betrayal had led him to Azkaban. Still under the influence of the Dementors that made him seek bloody revenge, Sirius finally found the willpower to escape Azkaban, using his Animagus form to elude the Dementors' notice.

Looking for Scabbers's trail, Sirius made his way to Hogwarts, using his knowledge of the secret passages to secretly infiltrate it. After failing to find Pettigrew hiding out in one of the secret rooms as he'd hoped, Sirius attempted to locate the Marauder's Map to get it to lead him to Pettigrew. He didn't find it in Filch's Office and assumed Professor Dumbledore may have found it and added it to his collection of magical trinkets; thus, he made his way into the Headmaster's Office through Wilkins's Passageway, where he was spotted by the Headpeople Portraits. With no Map in sight, and betting on the Hat's oath not to reveal what it saw in the minds of the people it sorted, Sirius put on the Sorting Hat and asked it if it had any clue where either the Map or Scabbers might be.

When this failed, he resorted to kidnapping the last known person to have seen Scabbers, Ron Weasley. He brought him to the Secret Classroom and was about to interrogate him when Hermione Granger, Harry Potter and Maximilian Candy caught up to him (having been informed as to his location by the Portrait of Hengist Rawkes). The misunderstanding was cleared up and Black was made to realize it would be spiteful and pointless to track down Pettigrew. The children then fetched Dumbledore, who offered Sirius Black a position as a new guardian in the Third Floor Corridor following the death of Gilderoy Lockhart's Pixies and the betrayal and subsequent departure of Kerbog.

Sirius was told the various mad events of the previous two years by Hermione Granger and settled in the third-to-last chamber of the Third Floor Corridor, which he turned into a cozy appartment for himself… whose furniture would, at the snap of his fingers, turn animate and ravenous. Having been informed of their status as the "Junior Marauders", Sirius contacted Fred and George Weasley and had them set the protections for the next chamber (the final one containing Tom Riddle and Quirinus Quirrell's shared Petrified body).