Koji is a sixth-grade boy who attends the same elementary school as Yuna. He lives with his younger sister Piko, who regularly bosses him around; despite his bratty demeanor, he is unable to stand up to her and often gives in to her demands. He can be selfish at times and carries a number of personal worries. Koji is skilled with computer games and has taken up combat karate, though he remains a white belt and is not as strong as Yuna. In private, he keeps a diary where he records his feelings, a hobby he keeps secret from others. During the second season, he develops romantic feelings for both Sae and Zuruko. Within the story, Koji appears as a supporting character and classmate of Yuna, interacting with her and the other children on Izayoi Island.