Rixie

Aika and the other mains apparently agreed to play "Hitori Kakurenbo", or "One-person Hide-and-Seek" (the way to do this is depicted in the movie, but if you're reaaaally curious, Google it. You're bound to find some stuff on it). Among the mains, though, only the one person went through with it, while the others backed out (note: they were communicating with each other, but as the others backed out, they were pretending to be going through with it). Then, you see the character that went through with it drown in her own bathtub and mysteriously disappeared. As the movie went on, it was found that the family members of this girl died one after another after that.

Scene switches to another woman, but I have no idea what she's supposed to be working as. Her boss/superior had apparently gotten some interesting information on this "Hitori Kakurenbo" thing, and told his woman to investigate. As it turns out, the information was actually an e-mail...written by Aika's character.

Aika and the other two who backed out had apparently been getting recurring dreams of the same thing - they were bound and unable to move, and every time they sleep, the woman in their dreams gets closer and closer to them, and this woman is holding a knife (if I remember right; Aika did this 'dream' scene). Believing that the woman in their dreams was their friend who was angry at them, they went to the house to investigate. Some series of events (which, really, I can't remember, but I vaguely recall that the other two mains died in that house and Aika managed to escape) led to Aika realizing that whoever played/had a part in "Hitori Kakurenbo" on the date the girl in the beginning played it (or something like that) will have this recurring dream... but the woman in their dreams was not their friend.

Rather, the woman was a previous inhabitant of that particular house. It was either the woman snapped one day or was extracting revenge on her husband/lover who cheated on her by killing him and burning the house down, laughing maniacally as she burned with it. I can't remember what was the reason that made this woman haunt the people who played "Hitori Kakurenbo", though. The working woman in the beginning starts to panic and called the number written in the e-mail, revealing that she, too, had a part in playing it that day. The phone was answered, but no one replied her. This woman tried not to fall asleep, but fails, resulting in her experiencing the recurring dream and presumably dying from it.

The movie ends with a line from Aika's e-mail that said something like this (alongside a creepy scene of the ghost-woman slowly walking down the steps of the working-woman's office):

"Every night, after I fall asleep, that dream keeps repeating, and that woman keeps getting closer. I've been trying not to fall asleep, but I don't think I can keep this up much longer. I'm sure the next time I fall asleep, I surely will..."