Student of class 3-1 who belongs to the literature club. He's aiming to become a novelist and respects Osamu Dazai. He generally has a calm and composed attitude. He initially is too embarrassed to show his novels to other people, but upon learning Akane's love for running and openness to use embarrassment as a method of improving oneself, he begins to open up to that prospect. From early childhood, Kotarō has received instruction in the performance of traditional dance and festival music accompaniment, Hayashi, at a local Shinto shrine.
He secretly starts dating Akane, but later reveals this to Takumi.
When Akane is forced to move to Chiba due to her father's work, Kotarō tries to get into the same school as her, despite his parents' objections, but is unsuccessful. He is unable to see Akane off on her moving day, but publishes an online story promising to love her forever. Despite their long-distance relationship, they are shown to still get in contact and visit each other. The final shot at the end of the anime shows them married with a kid years later.