R.A.B. in Harry Potter stands for Regulus Arcturus Black, the younger brother of Sirius Black. He was a member of the noble House of Black and was sorted into Slytherin at Hogwarts

. Regulus initially became a Death Eater, following Voldemort, but later turned against him after discovering the true cruelty of Voldemort’s methods. He learned about one of Voldemort’s Horcruxes, Salazar Slytherin’s locket, and stole it with the intention to destroy it. Regulus replaced the real locket with a fake one to deceive Voldemort but was killed in the process before he could destroy the Horcrux

. His identity as R.A.B. is revealed in "Harry Potter and the Deathly Hallows" when Harry, Ron, and Hermione find his name at 12 Grimmauld Place and learn from the house-elf Kreacher about Regulus’s attempt to undermine Voldemort by stealing the Horcrux

. In summary, Regulus Black (R.A.B.) is a pivotal but somewhat mysterious character who betrayed Voldemort by stealing a Horcrux, risking and ultimately losing his life to help bring about Voldemort’s downfall