They are characterized by their two mouths:
One being at her face, and the other being at the back of her head beneath the hair.
Often used in Japanese folklore and mythology.
It symbolize a person who eat a very small amount of food or just rarely eat anything.
One of the most well-known Futakuchi-onna stories tells of a man who chose his bride because she never ate a thing. Since he was a poor man, he could not bear the thought of feeding another mouth besides his own, so the fact that his wife never ate was a huge attraction. However, he soon noticed that his stores of rice were mysteriously being depleted. When he spied on his wife one day to see what was going on, he discovered that she was a Futakuchi-onna. He saw a gaping mouth open in the back of her head, and when she unbound her hair, it shoveled huge quantities of rice into the second mouth.