Affiliation: Death Eater (former)
Regulus, Sirius's younger brother, was a Death Eater during the last war against Voldemort. Though he's dead during the all the main action in the Harry Potter series, he plays an important role. In Harry Potter and the Half-Blood Prince, Harry finds that the Horcrux he and Dumbledore took from the ocean-side cave was a fake. Someone named R.A.B. had already stolen the original. At the start of Deathly Hallows, the identity of R.A.B. is still a mystery, but over the course of the book we find out that he was Regulus.
Regulus turned against Voldemort as a result of the Dark Lord's cruel treatment of the Black family house-elf, Kreacher. Regulus is one of Rowling's many examples that no one is beyond redemption and that anyone can choose to change (other prime examples include Severus Snape and even Draco Malfoy). Kreacher remains loyal to Regulus's memory, and during the Battle of Hogwarts, rallies the Hogwarts house-elves to fight against Voldemort. Kreacher's battle cry is "Fight the Dark Lord, in the name of brave Regulus!" (36.79).