In the half blood prince who do you think R.A.B is?

I don't think its Serius's brother for two reasons. First he wasn't an exceptional wizard. I say this because serius says he's not important enough to be killed by Voldemort; the "greatest" wizard alive was inches away from death after getting the fake locket so i doubt a half wit could have done it. Second Regulus died shortly after joining the death eaters. I don't think that Voldemorts closest followers knew that he had horcrux's (See Malfoy's reckless behavior with the diary) and if his closest followers didn't know i doubt someone who joined the death eaters for a short while then was killed would have figured it out, then broken through all of Voldemorts traps. I think that it was someone close to voldemort probably one of the kids in the room in the memory, Harry Dumbledore OLD slug-club members and R.A.B are the only ones who know about voldemorts horcrux's so far.