Rotogar said:
Almost certainly. I believe that's the only character we've ever heard of who might have those initials.
From my ex-GF (who's scarily into HP): Also, note that Regulus is believed to be dead, but, in either 4 or 5, Luna mentions a singer named Tuby Slugman or something who looks an awful lot like Sirius Black. This, perhaps, could be Regulus hiding under an assumed identity.
Now, how might Regulus have gotten through the defenses that nearly wrecked Dumbledore, the greatest wizard of his age, and his chosen apprentice? Simple...Regulus helped put them in place, and knew the back door that Voldemort left for himself.
My hypothesis is that Voldemort used Regulus, obviously a deep and abiding Death Eater, to assist him in hiding the Horcruxes*. At some point Regulus realized what was going on, decided that, despite his absolute hatred of mudbloods and half-breeds, that really, this Voldemort dude was a freak, and sabotaged some of Voldemort's Horcruxes to screw him over when he most needed it. Obviously, he didn't get all of them, as Tom Riddle's diary was one of them.
Now, here's a question...why didn't he go to the Order? Obviously, his brother was in it, and they would gladly accept someone with clues as to Voldemort's weaknesses. They accepted characters with less stellar credentials (*cough*Snape*cough*), so why not go to them? I'm currently thinking he's still a worthless git, but maybe he figured it was too important to risk his newfound anonymity and kept on the search.
Brad
* - When in Books-a-Million, I pulled out a Monster Manual to show ex-GF the D&D hippogriff, dragons, etc. I paused on the lich and noted that, really, that's what Voldemort was.