Seiya Tsukishima, a soft-spoken librarian devoted to literature, navigates fatherhood with a gentle, undemanding demeanor that starkly contrasts his wife Asako’s stricter parenting style. Married to Asako and father to Shiho and Shizuku, he supports his daughters’ creative pursuits—most notably Shizuku’s voracious reading and budding writing talent—yet remains largely unaware of the nuanced challenges and emotional undercurrents shaping their lives, embodying a passive yet affectionate presence.

His name, Seiya, intertwines the Japanese characters for "peaceful" and "to be," while his surname Tsukishima merges "moon" and "island," reflecting a calm, anchoring stability within the family. Though his direct narrative involvement is minimal, his profession as a librarian subtly mirrors the story’s thematic emphasis on storytelling and intellectual exploration.