The mysterious R.A.B., who turns out to be Sirius Black's brother Regulus, plays a small but pivotal role in the Harry Potter franchise.

Theory: R.A.B., who signed the letter Harry found instead of a Horcrux in Voldemort's cave, is Sirius Black's brother Regulus.

In a significant way, Order of the Phoenix is the brother novel to Prisoner of Azkaban.The third Harry Potter book is when Harry first learns of Sirius Black, and the fifth is when he has to say ...

Oh, and thanks to to a new theory, one fan thinks they know why Sirius never stuck tight to a fatherly role. Recently, a fan on Reddit hit up others asking them to give their Harry Potter theory a ...

Sirius is the son of the House of Black, a pure-blood family with a penchant for the Dark Arts. ... His love for Harry matched his love for James, like a brother.

Sirius, the oldest son and Black heir, ... “My parents with their Pure-blood mania, convinced that to be a Black made you practically royal … my idiot brother, soft enough to believe them. … ...