Harry Potter and the Deathly Hallows . Which Wizard is R.A.B.???

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"To the Dark Lord I know I will be dead long before you read this but i want you to know that it was i who discovered your secret. I have stolen the real Horcrux and intend to destroy it as soon as i can. I face death in the hope that when you meet your match, you will be mortal once more. R.A.B." R.A.B a character in harry potter that we already met??? Which Wizard??? Many speculates that it was Regulus black but as we all knew JKR love to surprise her readers, like the half blood prince we never suspected it was severus snape, we taught that it was a new character. I think same with R.A.B., maybe we already met this character. Any Idea who might it be???