Regulus Black is Sirius's younger brother. We don't actually see anything of him in Book 5, but we learn that he appears to have followed in his family's footsteps much more than Sirius. He joined Voldemort and then, when he felt that he was getting in too deep, he tried to leave. But Voldemort doesn't like people to leave him behind, so he murdered Regulus. Sirius notes bitterly that Regulus's name is still on the Black family tree, while his own has been blasted off.