Eiji Mishima is a fictional character central to protagonist Genki Horiguchi's life. A boxing coach by profession, he steps in as Genki's coach and father figure after the death of Genki's father. Eiji carries a troubled history, having once been a low-level yakuza underling marked by a back tattoo. This past led to societal exclusion and fueled struggles with alcoholism as his coping mechanism.

Despite these burdens, Eiji emerges as a highly skilled and dedicated boxing mentor, imparting his knowledge and experience to Genki. A profound bond develops between them, with Eiji providing crucial emotional support. Genki deeply admires his mentor and actively champions Eiji's romantic relationship with schoolteacher Yuko Ashigawa, whom Genki sees as a maternal figure.

Eiji and Yuko Ashigawa share a complex history, their connection rooted in childhood friendship that blossomed into mutual romance. Their relationship contends with societal rejection stemming from Eiji's background. Rivalry exists between Eiji and boxer Kenji Seki, encompassing both their boxing careers and affections for Yuko, though Yuko remains steadfastly loyal to Eiji.

Eiji's story concludes with his death from an unspecified illness. He passes away peacefully, expressing contentment at witnessing Genki's growth as a boxer and an individual.

Note: A separate, unrelated character named Eiji Mishima exists in the "Rurouni Kenshin" series. This version is a young boy from Shingetsu village who suffers family tragedy, seeks revenge, is adopted by Saito Hajime, and later joins the military.