Seiya Tsukishima is the father of Shizuku and Shiho Tsukishima, married to Asako Tsukishima. He works as a librarian at the local library, where his daughter Shizuku frequently borrows books. His responsibilities include managing library operations, such as overseeing the transition from traditional checkout cards to barcode systems—a change Shizuku dislikes. Seiya displays a gentle and permissive parenting style, often showing affection toward his daughters while remaining somewhat unaware of their daily experiences and emotional difficulties. This approach contrasts with his wife Asako's more vigilant parenting.

He appears supportive yet not deeply engaged in Shizuku's creative projects or academic stresses. For instance, during Shizuku's intense focus on writing her novel—which sparks family tension and declining grades—Asako and Shiho take more active roles in confronting these issues. Seiya's interactions primarily occur in domestic or work environments, such as accepting lunch delivered by Shizuku at the library or sharing casual conversations at home. His character remains consistent throughout the narrative, maintaining a caring but peripheral role within the family without significant development or expanded backstory.