Kouki Nitani is a boy who first appears as a nursery school classmate of Rin Kaga and quickly becomes her closest childhood friend. He lives alone with his single mother, Yukari Nitani, because his parents divorced when he was very young and he has no memory of his father. Their household mirrors Rin’s situation with Daikichi, creating an early bond between the two families.

In the early part of the story, Kouki is an energetic and mischievous child who often acts out and dislikes being bored or told to sit still. He can be bratty and resistant to adult authority, yet he readily listens to Rin and cares about her feelings. He is fiercely loyal to her and often drags her into playful adventures. He admires Daikichi as a cool uncle figure and sometimes tries to imitate him, though his attempts usually dissolve into noisy chaos. His personality reflects a lively, slightly rough-around-the-edges elementary schooler whose rowdiness masks a genuine soft spot for the people he loves.

As the series progresses and the children grow into teenagers, Kouki’s role deepens. He develops romantic feelings for Rin and hopes for a deeper relationship with her. However, he comes to understand that Rin’s affections lie elsewhere—with Daikichi. Rather than acting on jealousy or resentment, Kouki matures significantly and chooses to support Rin. He actively works to help her confess her feelings to Daikichi, consistently putting her happiness above his own. This selfless act highlights his growth from an impulsive child into a considerate young man.

Kouki’s key relationships center on his bond with Rin, which evolves from innocent nursery school friendship to a more complex teenage attachment, and his connection to Daikichi, whom he views as a role model and reliable adult presence. Through his mother, Yukari, he also provides a bridge that brings the two single-parent families together. His main development lies in learning to temper his childish willfulness and in ultimately letting go of his own desires for the sake of someone he cares for deeply. While he possesses no extraordinary talents, his emotional awareness and capacity to put Rin first become his most notable qualities, shaping a quiet but meaningful arc within the story.