The mysterious initials left on the taunting note to Voldemort hidden inside the fake locket Harry and Dumbledore found in the cave must belong to Regulus Black, younger brother of Harry's godfather, Sirius. Sirius tells Harry that Regulus was a death eater, but was killed after deserting Voldemort's ranks and that he lasted only a few days after leaving -- enough time to steal the real horcrux, replace it with a fake one and hide the real one somewhere (the trio saw a "heavy" locket while cleaning at the Order's headquarters -- the Black family home -- 12 Grimmauld Place). As Sirius explained to Harry, you don't just stop being a death eater, "It's a lifetime of service."

It's unlikely J.K. Rowling will be introducing anyone we haven't already met in some way -- not at this stage. There would be no point. Harry needs to understand the recent past in order to understand his present. A relative of Lavender Brown's or Katie Bell's are good guesses, but for the most part, JKR has structured her stories in such a way that the mystery is something that's generally right under our noses all the time or appear in one book only to make sense or become integral in another. The answers are right there, we just can't piece them together (or at least Harry can't). Rarely does she spring someone on us who we don't already know about in some way. Barty Crouch Jr. is a perfect example. In the beginning of Goblet of Fire, the story of Crouch Jr. was merely a cautionary tale told to Harry to illustrate a father's willingness to break rules for a son, and the terrible price it cost him. Then the dude ends up being the key to the whole freakin' plot. Regulus has been introduced to us in much the same way. I'm almost 100 percent sure he's totally dead, but I know that the information surrounding his life and death will be vital -- though maybe not quite as shocking as the Barty Crouch Jr. revelation. We might even meet him. I just don't know how. (I'm thinking time turner, maybe?)

While it's obvious that Bell and Brown have parents, we haven't heard about them in any specific detail or been led to believe that they are in any way important, unlike a few Slytherin children. JKR has been very explicit with the names of her death eaters, so we can clearly identify who among them are parents of Hogwarts students.

Galadriel Waters, author of the "Unofficial Guide to the Mysteries of Harry Potter" outlines basic rules for sleuthing the mysteries of the Harry Potter books, but this one is applies here:

Rule number 3: There's no such thing as coincidence.

JKR has given us a character with names that fit two out of the three initials. Why mention Regulus at all if he weren't integral to the plot in some way? JKR mentions very little in passing. Literally, everything is important, even when you think it isn't. Waters reiterates that the reader must excercise "constant vigilance!"