Hanako-san

Also known as Toire no Hanako-san or hanako of the toilet. She is said to be a young girl from the World War II era that haunts the restrooms all over Japan.

How to Summon Her
It is said that in order to call her fourth you must go to the third flour girls bathroom and knock on the third stall door and say "Are you there Hanako-san?" The voice of a young girl is said to answer you with "I'm here." When you open the door you will see a little girl in a red skirt (some say she has a bobbed hair cut.) This is one of the most common outcomes.

Variations
What happens after you perform the ritual varies however in different parts of Japan.

Yamangata Prefecture -   Upon entering the stall, the caller will suddenly be eaten by a three headed lizard creature that was mimicking the girls voice.

Iwate Prefecture -  A large white hand is said to emerge from the door after she is called.

Kanagawa Prefecture - Once she is called a bloody hand will appear and lash out at you.