Futakuchi-onna (Aku)

"Futakuchi-onna (ふたくちおんな, "two-mouthed woman") is a type of yōkai or Japanese monster. She is characterized by her two mouths – a normal one located on her face and a second one on the back of the head beneath the hair. There, the woman's skull splits apart, forming lips, teeth and a tongue, creating an entirely functional second mouth."

"In many stories, the soon-to-be futakuchi-onna is a wife of a miser and rarely eats. To counteract this, a second mouth mysteriously appears on the back of the woman's head. The second mouth often mumbles spiteful and threatening things to the woman and demands food. If it is not fed, it can screech obscenely and cause the woman tremendous pain." -- Futakuchi-onna WIKI