Sai serves as Hiwou's older brother within the Machine People, a clan dedicated to crafting clockwork dolls for peaceful celebrations and upholding their strict non-weaponization principle. His family includes siblings Hiwou, Mayu, Tetsu, and baby Jyoubu, navigating life after their father Masurao's departure and their mother's death before the story begins.

Struggling with self-doubt regarding his mechanical skills compared to Hiwou, Sai initially hesitates during critical operations of their giant clockwork doll, Homura. Despite this insecurity, he possesses fundamental knowledge essential for Homura's maintenance and operation.

When the Wind Gang destroys their village, Sai flees with his siblings and friends Shishi and Machi. Their journey becomes a quest to find their father while evading the Wind Gang. Repeatedly forced to operate Homura during conflicts, Sai's technical proficiency and confidence incrementally grow through direct experience and necessity.

Sai prioritizes protecting his younger siblings. His relationship with Hiwou features initial friction, his cautious pragmatism clashing with Hiwou's bold impulsiveness, yet evolves into mutual reliance during shared struggles against the Wind Gang.

Sai's character arc traces a path from persistent insecurity toward greater self-assurance, forged through maintaining and piloting Homura under pressure. By the story's end, he maintains his technical contributions to the group's efforts without succumbing to his former self-doubt.