Despite his outstanding talent for wizardry, Sirius was never properly instructed, and spent years using his talent for burning things and earning a living.
A few years ago, the elder Librarian Nazar adopted him as a disciple, hoping to teach Sirius some discipline and wisdom.
However, Sirius' fascination with fire was far too great, and after a fire that caused the loss of three incunable tomes, Nazar treated him as a lost cause and expelled Sirius from the Library.