Daigorou (だいごろう) is the name of one of the Kokeshi dolls that come into Shun's room. He is the one that kills Mikami. He has a sarcastic, husky voice, and says very negative comments, as well as insulting Shun, Mikami and Yuu.
Once Shun wins the game, he leaves the room along with Kobayashi. 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.