Could this be Sirius's brother, Regulus? They could have some middle name no ones heard of yet.
In the letter in the horcrux, the letter was adressed to the "Dark Lord', which only Death Eater's called Voldemort. Wasn't Sirius's brother an ex-death eater?
Voldemort supposedly killed Regulus himself, maybe it was because he found out that Regulus knew about the horocrux's.
When Sirius told Harry that Regulus joined the Death Eaters, he didn't say it in a hating way, just in a regretful way.
I really think I'm right here, lmk!

So let me guess. You googled something like "R.A.B." and "Harry Potter" ? And you came up with this, and now it is your idea?

that actually sounds pretty good, nice insight.

Lol phuck you I thought of this myself when I read the book.
I'm not THAT desperate for attention man...

Anyone else have any thoughts?

My thought is to just wait until the book comes out. I ruined the 6th book because i was too curious on who was going to die and didnt like it one bit.

i thought so

i love Harry potter 6