Kōichi Tanemura is a 16-year-old student and Ohana Matsumae's childhood friend from Tokyo. They attended the same middle school and shared a close bond before her sudden move to her grandmother's hot spring inn, Kissuisō. Kōichi is kind, patient, and notably indirect in his decision-making, a trait Ohana observed. Before she left, he confessed his romantic feelings to her but ran away immediately after, avoiding her response.

Despite the distance, Kōichi's feelings persisted. He attempted to visit Ohana at Kissuisō but turned back without meeting her after seeing chaotic scenes at the inn. When Ohana later visited Tokyo, they reunited. During this time, a female coworker confessed her feelings for Kōichi; he declined, stating he had someone else in mind. Their interactions briefly revived their old dynamic, though Ohana departed abruptly. Kōichi sensed Ohana's deepening connection to her new life at Kissuisō might exclude him, yet his affection remained steady.

A significant moment came when Ohana, realizing her own feelings, tried to express them over the phone. Kōichi seemingly rejected her with a casual "See ya," suggesting uncertainty about future meetings. However, episode 23 clarified this wasn't a final rejection. Seeing a video of Ohana working diligently at Kissuisō reinforced his enduring feelings, reminding him of her qualities. By the series' end, their relationship evolved into mutual romantic commitment, and they began dating.

His given name, "Kōichi," combines "filial piety, obedience" (孝) and "one" (一). His surname, "Tanemura," comes from "seed, species, trick" (種) and "town, village" (村). Ohana uses the affectionate nickname "Kou-chan," reflecting their deep bond.