Ashido is the mysterious Shinigami from Bleach who first appeared on episode 148. Not much is known about him except from the fact that his name is Ashido, and that he stayed inside Hueco Mundo to slay Hollows before they get to the real world to lessen the burden on the other Shinigamis.
He came to Hueco Mundo because he and his squad were chasing after some Hollows. They followed them to their territory. When they got there they continued slaying until only Ashido was left. He wears a Hollow’s mask because it was the mask of the last Hollow his friend killed before he died. The Hollow’s mask hides his handsome face but it helps him in battles, it deflects Hollow’s Cero blast and is a very good shield.