Shigeru Shikishima, a childhood friend and neighbor of protagonist Neri Ooishi, shares lifelong family ties marked by early physical familiarity—including infant bathing rituals—that forged deep trust between them. As a trained osteopath, he specializes in athlete care, particularly Neri's well-being. His therapeutic manipulations occasionally elicit audible reactions, sparking misconceptions about their relationship. A critical misunderstanding arose when he treated Neri post-volleyball collision in a restroom; resulting rumors damaged his school reputation.

Consequently, school authorities assigned him as Kokuyodani High's girls' volleyball coach under disciplinary framing. Initial team unease stemmed from rumors and gender concerns, yet he prioritized athlete health and recovery with unwavering dedication. His protective commitment to Neri transcends duty, informed by awareness of her family trauma, including her sister Mari's death.

Characterized by generosity, emotional restraint, and saintly selflessness, he supports others financially, physically, and emotionally while maintaining strict professional boundaries. His sister Michiru Shikishima exists, though their dynamic remains undefined. Throughout Neri's struggles with volleyball aggression and self-acceptance, Shigeru provides stability, guiding her to harmonize competitive drive with teamwork.