There is always a supernumerary student in the Class 3 Grade 3. They do not know who this guy is, as their memories were changed in Another Anime. Some of them will be dead because of this supernumerary one, which is called the “curse” by them. Some one advised that if one of them could be segregated and the curse might be stopped. It means that they can't talk to this guy and ignore this guy completely.
As the pronunciation of “Misaki” is the same to the guy who died 26 years ago, Misaki is segregated by them. Kouichi did not know this rule firstly and he talked to Misaki. Two students were dead after then, and Kouichi is segregated too in Another Episode 6. As the same encounter they have, Misaki and Kouichi become good friends.