User blog comment:Xilinoc/Everything but the Rain op. 4: Dark of the Bleeding Moon/@comment-5551986-20130328234437

Isshin said he encountered an Imperect Arrancarr before. I'd take a guess that this hollow is it. As to its hole closing i have a theory. Arrancarr are hollow who have acquired or attempted to acquire, shinigami powers, maybe Aizen thought the closing of the hole rather than the removal of the mask was key to this. I mean from some sort of view, when Shinji and his buddies were hollowfied, they got masks which would indicate they have hollow powers, if for some reason Aizen thought the masks were the source of a hollows powers, why would he want to remove it? The colour? Im guessing that he thought it would be associated with shinigami, hollow are white, shinigami are black (no pun intended in anyway, shape or form) Hell. It might have a proto-hierro, its reiatsu might be black as a guess, it might just be physically compressed black reiatsu. Put everything bluntly... Hollow = Hole Shinigami = no hole, Mask on Shinigami = Hollow powers leaving the mask on, would suggest their power stems from this, so he would attempt to keep it.