Kōki Nitani is the only son of Yukari Nitani and a childhood friend of Rin Kaga. He meets Rin at nursery school when they are six years old, and the two families quickly become close after Daikichi Kawachi meets Yukari. Kōki grows up in a single-parent household following his parents' divorce, so he has no memory of his father. He lives alone with his mother, and this shared experience of being raised by a single parent forms a natural bond between his family and the household that Daikichi and Rin build together.

In his early years, Kōki is portrayed as an energetic and often mischievous boy. He is described as a troublemaker who frequently has difficulty listening to adults, yet he displays a protective side toward Rin and is easily swayed by her wishes because he genuinely cares about her feelings. He is immature and bratty at times, but these traits are balanced by a cheerful disposition and a growing sense of loyalty. His behavior during childhood is typical of a healthy, active boy, and he often acts as a contrast to Rin's more calm and reserved nature. He walks to school with Rin hand in hand, a routine that persists even as they grow older and that reflects the closeness of their relationship.

As a teenager, Kōki matures considerably, though he retains his playful, affectionate dynamic with Rin. He continues to walk to school with her every morning and they interact like close siblings, teasing each other and sharing a comfortable rapport. Over time, Kōki develops genuine romantic feelings for Rin, and this becomes a central thread in the second half of the story. He struggles with the emotional complexity of adolescence and the pressures that come with his long-standing bond with her. His feelings for Rin are not immediately reciprocated in the same way, and the story explores how he navigates this unrequited affection while preserving their deep friendship.

Kōki's relationship with Daikichi is also significant. Because Kōki grows up spending a great deal of time at Daikichi's home, Daikichi becomes a secondary male role model for him. They eat together, Daikichi cares for Kōki when he is hurt, and the boy is treated almost like a member of the family. Kōki looks up to Daikichi and respects him, and this bond reinforces the sense of an extended, unconventional family. His relationship with his mother, Yukari, is close and supportive, though she is often the one managing his energetic behavior during his younger years.

Physically, Kōki has short, spiky black hair and sharp brown eyes. During his childhood, he is noted to bear a slight resemblance to Daikichi, despite having no blood relation. As he grows into his teenage years, his appearance comes to resemble his mother's more closely.

Kōki does not possess any extraordinary abilities or talents that set him apart in a dramatic way. His strengths lie in his emotional availability, his loyalty to those he loves, and his willingness to be there for Rin during moments of need. He is academically capable and applies himself to his studies as he gets older, but his most notable trait is his capacity for emotional growth. He evolves from an impulsive, rambunctious child into a young man capable of deep feeling and self-awareness, even when those feelings are not returned in the way he hopes. His development reflects the broader themes of the story about growing up, forming bonds, and learning to handle complex emotions within the safety of a loving, unconventional family structure.