Kōichi Tanemura is a sixteen-year-old high school student and a secondary character in the story. He grew up in Tokyo and attended the same middle school as Ohana Matsumae, the main protagonist, becoming her closest friend. His background is that of an ordinary urban teenager, described as a gentle, city-dwelling boy who is considerate toward others.

His personality is kind, patient, and somewhat roundabout in his approach to decisions and emotions. He is a sincere and polite young man who tends to put the happiness of those he cares about above his own desires. This thoughtful nature makes him a reliable friend, though his indirectness sometimes leads to hesitation in expressing his true feelings.

Kōichi's primary motivation is his deep affection for Ohana. He wants to support her and see her thrive, even when they are separated by distance. Before Ohana leaves Tokyo to live at her grandmother's hot spring inn, he confesses his love for her. However, he becomes nervous and runs away before she can respond, leaving his feelings unresolved for much of the series. His actions are driven by a desire to be honest with Ohana while also fearing rejection or disrupting their friendship.

In the story, Kōichi serves as a connection to Ohana's former life in Tokyo and as a romantic interest. He appears intermittently throughout the series, often communicating with Ohana through text messages and phone calls, and visits her at the inn on occasion. His role is that of a supportive figure who provides emotional grounding for Ohana as she navigates the challenges of working at Kissuisō and adapting to a new environment.

His key relationship is with Ohana Matsumae. Their bond begins as close friendship and gradually deepens into romantic love. Kōichi is patient with Ohana's energetic and sometimes chaotic personality, and he consistently encourages her. Ohana, in turn, comes to realize her own feelings for him after his confession. Near the end of the series, she responds to his confession, affirming that she loves him as well, solidifying their mutual affection. The relationship is portrayed as a steady, heartfelt connection that withstands the physical distance between them.

Kōichi undergoes subtle development over the course of the story. Initially, he is hesitant and avoids direct confrontation about his emotions, choosing instead to act in a roundabout manner. As the series progresses and he continues to support Ohana from afar, he grows more confident in his feelings and learns to be more open. By the time Ohana finally responds to his confession, he has matured in his ability to accept and express his love without running away.

He does not possess any notable supernatural or special abilities. His strengths lie in his emotional intelligence, empathy, and unwavering loyalty as a friend and partner. He is an ordinary teenager whose development is centered on personal growth and the courage to be vulnerable.