Ok
Have been looking into this one - here are your candidates
Regulas Black - Killed by Death Eaters for trying to back out. A good candidate because we learn in GOF that all of the Death Eaters knew the steps Voldemort had taken to ensure his immortality. Sirius believed he got in too far and tried to back out and that was why he was killed. Uncle's name was Alphard which may be Regulas's middle name. Regulas is the only charracter thus far that has been linked with the initials RB. But the tapestry at Grimauld Place does not give us his middle name. The letter written in HBP is very keen that the Dark Lord (a term only used by Death Eaters) knew who discovered his Horcrux. In Order of the Phoenix, a locket was discovered at Grimauld Place that no-one could get into...
Burke or Borgin - We don't know either of their first names yet and could be your second favourite candidate after Regulas. Hermione mildly attempted to purchase a necklace in HBP and we know that Burke would have been very keen to have the locket back after whatshername died.
Mrs Bones - Mother to Amelia Susan Bones, Grandmother to Susan Bones. Not sure about this one as I think they said her first name was something else - can anyone remember I have now lent my book to someone else and cannot check!
Bagman - Father of Ludo and Otto. Don't have much info on him from the books I'm afraid...
Anyone else that can be added? I doubt JK will introduce new characters in the final instalment.