Tsutomu Ichinose is Kozue Ayuhara's cousin and her vital emotional anchor. He serves as vice-president of Fujimi High School's student council while actively participating in the newspaper club, a role enabling him to closely document and champion Kozue's athletic endeavors.

Their bond deepens beyond family, evolving into mutual, largely unspoken romantic feelings. This connection makes him Kozue's primary stability against intense athletic pressure and personal conflict. His support sometimes takes the form of harsh guidance, including physical interventions like slapping, intended to propel Kozue forward during moments of doubt or perceived weakness.

Forced by financial hardship to leave school prematurely, Tsutomu begins working at his father's fruit shop and later drives a delivery van to support his family. This shift strains his relationship with Kozue, especially when she tries to assist him financially; he rejects her efforts, interpreting them as pity rather than genuine support.

His story ends tragically across adaptations. The manga depicts his death after being struck by a train. The anime presents a fatal mountain van accident during delivery work. The live-action drama offers a third version, a fatal car collision. Crucially, Kozue misses the chance for a final farewell each time, prioritizing her commitment to ongoing volleyball matches over reaching him before his death.

His influence profoundly extends beyond death. Kozue discovers his diary, revealing his enduring belief in her potential and his desire for her to become the world's premier volleyball attacker. This discovery reignites her motivation after a period of grief-induced withdrawal. The diary acts as a posthumous conduit for his support, directly catalyzing her return to competitive play and renewed pursuit of excellence.