Final answer:
The initials R.A.B. belong to Regulus Black, who is a character in the Harry Potter series known for turning against Voldemort and attempting to destroy one of his Horcruxes.
Step-by-step explanation:
The initials R.A.B. belong to Regulus Black. This is revealed in the Harry Potter series. Regulus Arcturus Black was a member of the Black family, a family that was notorious for being full of dark wizards and supporters of Voldemort. However, Regulus eventually turned against Voldemort and made it his mission to destroy one of Voldemort's Horcruxes. The locket Horcrux in particular was discovered by Harry Potter and his friends in 'Harry Potter and the Half-Blood Prince' with the enigmatic initials R.A.B. inscribed on a note found with the locket, eventually leading them to uncover the identity behind these initials.