Sayo is a classmate and close friend of Atsumi Miyagawa, expressing an intense fondness for cute things. She frequently showers the slightly older Atsumi with physical affection and embraces throughout the day, treating her like a child. Sayo persistently encourages Atsumi to consume rich foods and milk, believing it might address her stature.

Learning that Atsumi has a younger brother named Atsushi, Sayo immediately assumes he must be adorable. However, encountering him without knowing his identity, she perceives him as an attractive young man and develops a romantic interest, imagining scenarios with him. Discovering his true status as an elementary school student complicates her interactions. Despite this revelation, her feelings do not completely fade, creating ongoing internal conflict as she struggles to reconcile her emotions with his age and maturity. These romantic inclinations persist across subsequent storylines, including moments of private grappling with unresolved feelings.

In later developments, she continues interacting with both siblings, such as seeking shelter from rain with Hina at Atsushi's house. These encounters highlight the awkward dynamic fueled by her lingering, largely unreciprocated affection, further complicated by his childish demeanor and preoccupations. Her attempts to navigate this emotional tension continue to shape her interactions without clear resolution.