The Hari-onna appears as a beautiful woman with very long, silky black hair. She has full control of her hair, which turn out to be fully mobile, each serpentine strand tipped with a sharp, barbed hook
The Futakuchi-onna has, on the back of her head beneath the hair, a huge fully formed mouth. The skull litterally splits apart, forming lips, teeth and a tongue. It speaks with a mind of it's own and has it's own hungry will. The Futakuchi-onna's hair tends to be very long, either depicted as forming two giant snake-like masses or instead hand-shaped tendrils.
The Hari-onna is usually said to be praying on young men, laughing at them. One could presume that she is the spirit of a scorned woman, who may have been turned down by a man - or whose lover had an affair with another woman.
The Futakuchi-onna is often depicted as the victim of a curse that grows the mouth on the back of their head and transforms their hair. For this reason, she isn't as much an angry spirit as a sad, tortured one.
The Hari-onna is fond of tormenting and preying upon young men. Appearing on roads as a beautiful woman, she laughs derisively at whoever strikes her fancy, and if he makes the mistake of laughing back, she lets down her deadly hair and gives chase.
The mouth on the back of the Futakuchi-onna's head will mumble spiteful and threaten things to itself, and demand food. If it is not fed, it will screech obscenely and cause the woman tremendous pain. She often appears as a beautiful (normal) woman who doesn't eat much or at all - until someone sneaks on her and discovers her second mouth eating twice as much as a normal Human being's, using her hair.