Kazuhiko is Masaki's cousin and happens to be the same age as him. He works at a ryokan back in their hometown.
The two meet the Fujiyoshi family when Masaki comes back home to pick up a letter from his uncle. His uncle had already tipped off Kazuhiko about the visit, so he was ready to put up Masaki, his family, and their friends at the ryokan where he works.
Content compiled by AnimeList.moe from publicly available sources.
