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When Kanon's story begins, Yuichi meets the five heroines and becomes intrinsically linked with each one of them as the story progresses. Yuichi's main purpose in the story is to be the one where the story revolves around as the protagonist and thus he becomes deeply rooted into each of the five main plot lines associated with each of the heroines.
Yuichi attempts in each arc to stay with the respective girl until their arc has concluded. With Mai, Yuichi helped to fight off the demons, and with Makoto he stayed with her until she ultimately died. With Shiori, he indirectly prevented her from committing suicide, and with Nayuki, he remembers when Nayuki confessed to him 7 years ago with a snow bunny.
He also tried to be there for her when her mother was in the hospital. Lastly, with Ayu, Yuichi tried to help her find her lost item, eventually finding it and repairing it with Nayuki's help.