Chiyo Fujii, elder sister to Chisato Fujii and first daughter of Fujii, is a former Seishuu classmate of Goro Shigeno. She shares an approximate age and year level with Daigo Shigeno and Mutsuko Sakura. Physically, Chiyo sports eyeglasses and short twin-braided hair, maintaining a perpetually stern expression. Her notable height of 180cm makes her the tallest member of her school club, earning her the moniker "Huge Schoolgirl."

Chiyo played baseball alongside her younger sister Chisato until fourth grade before quitting for unspecified reasons. After leaving the sport, she embraced indoor pursuits, joined the art club, and identified as non-athletic. However, her connection to baseball unexpectedly resurfaced through her feelings for a player named Nishina. This led her to become the manager of the Fuurin Academy Middle School baseball team, providing logistical and motivational support despite her self-professed lack of athleticism.

Chisato admires her older sister, having started baseball to emulate Chiyo, reflecting a dynamic of "Big Sister Worship." From Chiyo's perspective, Chisato occasionally manifests as an "Annoying Younger Sibling," though the sisters maintain a close bond.