Hanako-san

Hanako-san is an urban legend about the spirit of a young girl who haunts school bathrooms. Her origins vary from different sources, some saying that she is the ghost of a girl killed during an air raid in World War II, while others say that she was murdered by a psychotic parent or stranger.

According to the legend, a person who goes to the third stall in the girls' bathroom on the third floor can have a chance of meeting Hanako-san. They are supposed to knock three times before asking "Are you there, Hanako-san?" Then they will hear a voice answer, "I'm here." If the person chooses to enter the stall, there will be a small girl with bobbed hair in a red skirt.
Fortunately, she is relatively harmless and only has her goal set on scaring people. She might scream loudly, or open and close doors extremely fast while throwing around toilet paper, but that's about it. The worst thing that has happened in some variations of the story is that she shoves the victim's head in the toilet.

In other words, she's basically the Japanese version of Bloody Mary, with very few exceptions. Hanako-san is a popular and widespread urban legend in Japan, and school children play it often as a rite of courage. Sometimes students will even dare each other to do it in the middle of the school day.

Many people have written first-person stories about Hanako-San to make them look like eye-witness reports, even though they are almost entirely made up. This kind of writing is similar to reports written about Slenderman and Kunekune, where they write stories to make it seem realistic and terrifying.

So maybe if you see Japanese children in shows or a book or something like that playing this game, now you know what they're actually doing. Maybe if you're still in school, you can even play Hanako-San yourself.