Kobayashi (こばやし) is the name of one of the Kokeshi dolls that come into Shun's room. He is the one that kills Yuu and also the one that talks the most. He has a well-tuned, high-pitched voice.
Once Shun wins the game, he leaves the room along with Daigorou. He probably dies because he didn't manage to turn back into a human and get into the cube on top of the hospital.
We don't know who he really was.