Kōzō Samejima coaches the Fuji Daiichi Boys football team while Nozomi Onda is in middle school. He is an older man with neck-length dark-brown hair swept to the sides, dark-colored eyes, a scraggly mustache, and stubbled facial hair. His coaching style is serious and strict, yet reveals underlying care for his players' well-being.

Following Nozomi's injury during a game, Samejima barred her from official matches, citing physical disparity risks between male and female players. Others interpreted this viewpoint as reflecting sexist assumptions about women's physical capabilities. However, he permitted her participation in a newcomers' tournament to assess her abilities firsthand.

After observing Nozomi's performance and technical skill, Samejima acknowledged his initial misjudgment and recognized her potential to excel in girls' football. He advised her to seek out a girls' soccer team to revitalize the sport, urging her to "make girls' soccer the best in the world again" and explore football's fundamental appeal as a restrictive yet captivating sport.

Samejima maintained a professional relationship with Nozomi, rooted in recognizing her talent while prioritizing safety concerns. His development shifts from restrictive oversight to active advocacy for her future in women's soccer.

His surname, Samejima, combines the Japanese words for "shark" and "island." His given name, Kōzō, signifies "happiness, good luck" and "construction" or "physique."