Yoko Kawashima is engaged to Yoko Akimoto, Haru Nonaka's mother, making him Haru's stepfather. He works as a professor at the same college where Yoko Akimoto teaches. Kawashima exhibits an easygoing and understanding personality, demonstrating genuine care for Haru's mother and making consistent efforts to build a relationship with Haru despite their non-traditional family dynamic.

Physically, he is an elderly man of average height with short white hair. He typically wears reading glasses and is often seen in a white dress shirt paired with slacks.

Within the family structure, Haru lives independently but near her mother and Kawashima. Interactions between them remain infrequent, reflecting a distance maintained primarily by Haru. Kawashima, alongside Haru's mother, expresses concern during Haru's temporary relocation to Ise in Mie Prefecture to live with her grandmother. Haru's mother notifies Haru's employer about this absence, indicating shared concern within the household. Following Haru's return to Tokyo, the established dynamic of limited contact continues. Kawashima respects Haru's preference for independence while remaining part of her extended familial context.