From PART TWO of Interview with JKR by Emerson and Melissa
JKR: Ohhh, good.
JKR: No, I'm glad! Yes?
MA: Can we figure out who he is, from what we know so far?
[Note: JKR has adopted slightly evil look here]
JKR: Do you have a theory?
MA: We've come up with Regulus Black.
JKR: Have you now?
JKR: Well, I think that would be, um, a fine guess.
MA: And perhaps, being Sirius’s brother, he had another mirror –
JKR: [drums fingers on soda can]
MA: Does he have the other mirror, or Sirius’s mirror —
JKR: I have no comment at all on that mirror. That mirror is not on the table. [Laughter from all; Jo's is maniacal.]
MA: Let the record note that she has drummed her fingers on her Coke can in a very Mr. Burns-like way.
ES: What's one question you wished to be asked and what would be the answer to that question?
JKR: Um — [long pause] — such a good question. What do I wish I could be asked? [Pause.] Today, just today, July the 16th, I was really hoping someone would ask me about R.A.B., and you did it. Just today, because I think that is — well, I hoped that people would.
MA: Is there more we should ask about him?
JKR: There are things you will deduce on further readings, I think — well you two definitely will, for sure — that, yeah, I was really hoping that R.A.B. would come out.
MA: Forgive me if I'm remembering incorrectly, but was Regulus the one who was murdered by Voldemort —
JKR: Well Sirius said he wouldn't have been because he wasn't important enough, remember?
MA: But that doesn’t have to be true, if [R.A.B.] is writing Voldemort a personal note.
JKR: That doesn't necessarily show that Voldemort killed him, personally, but Sirius himself suspected that Regulus got in a little too deep. Like Draco. He was attracted to it, but the reality of what it meant was way too much to handle.