The initials inscribed on the locket Dumbledore believed to be holding one of Voldemort’s horcruxes is a point of speculation. Most believe it is the initials of a person, perhaps someone who figured out the secret of the locket before Dumbledore. Most believe the initials stand for Regulus Black, the brother of Sirius Black, thought to be dead, killed by Voldemort’s death eaters for betrayal years ago.
The mystery surrounding the letters is of great debate. It is assumed they are the initials of a person. However, most of the initials and anagrams JK Rowling uses in her books are not related to a person. DA, OWLs, NEWTs, SPEW are all initials for an organization or a level of achievement at Hogwarts. None, aside from the D in Dumbledore’s Army, represent a person. And even that may be a point of contention, because the D in DA represented many things.
RAB could very well be the initials for an evil organization started by Voldemort’s new army of death eaters. What it actually stands for is still a mystery, and probably of little significance.
However, if they are the initials of a person, there are several possibilities outside of Regulus Black.
First, there is Lavender Brown. Perhaps the RAB is related to Lavender, Hermione Granger’s roommate at Hogwarts and Ron Weasley’s romantic interest at one point.
She is also a pure-blood witch, meaning she has deep ties to the wizarding world. It’s also an indicator that her family may have sympathized with Voldemort’s cause and joined his dearth eaters. Perhaps one of them was trusted enough to retrieve the horcrux before Dumbledore could get to it.
Another is Cuthbert Binns, the ghost history of magic teacher. It’s a stretch that he has any ties to Voldemort or death eaters. But it’s not out of the realm of possibility that one of his descendents joined up with the death eaters and gained a level of trust with Voldemort that few have.
There is not much known about Katie Bell, except that she is a year older than Harry and plays on the Gryffindor quidditch team. Little is known abour her lineage or her allegiances. But she joins Dumbledore’s Army in an attempt to learn defense against the dark arts. Perhaps she is related to the mysterious RAB, but there is not enough information about her to make a legitimate argument.
If RAB is a person, it is most likely related to the Black family. There are a number of Blacks, as illustrated in the family tree at 12 Grimmauld Place, with loyalties to Voldemort and the Order of the Phoenix. It just seems too obvious that the initials stand for Regulus Black. He’s probably dead anyway.
My guess is that it stands for Resurrected Army of Blood, or something, and it represents a new legion of death eaters, much like DA represents a new Order of the Phoenix.