Ok, does anyone have clues as to who this person is?

This person stole the 3rd Horcrux in book 6. I have gone through a list of people, and the incials don't match anyone.

Also, in The Order of the Phoneix, wasn't there a necklace, locket type thing found in Head Quarters, when they were cleaning up the place. It was mentioned that it was impossible to open, that it might have had a sticking charm on it simular to the one on the back of Sirius' Mother's Portrait.