Shiina Amamiya transfers into Hakujō Academy during her second year, having previously lived in Otaru, Hokkaido – a period she seldom discusses. Her move is partly driven by the academy's renowned music teacher, the "Performer of the Sunset." A skilled pianist who placed second to Haruka Nogizaka at the London Piano Contest, Shiina also possesses proficiency in naginatajutsu. Her consistently positive, cheerful, and sociable nature eases her transition into the school.

Romantic feelings for Yuuto Ayase develop following multiple coincidental encounters before her transfer, incidents often resulting in awkward situations like accidental panty exposures. These lead to her early reputation as an "Accidental Pervert" magnet around Yuuto. Despite her usual self-confidence and proactive demeanor, Shiina exhibits shyness and hesitation in romantic matters. Encouraged by friends Ryōko and Mai to pursue Yuuto, she remains conflicted and sometimes facilitates moments between Yuuto and Haruka instead.

Her character arc involves navigating unrequited feelings while sustaining friendships. She acknowledges Yuuto's deep connection with Haruka, ultimately prioritizing their happiness through an "I Want My Beloved to Be Happy" ethos, refraining from sabotaging their relationship despite her own affections. Shiina actively participates in school events like swimming tournaments and cultural festivals, showcasing her competitive spirit and teamwork. During one swimming competition, she suffers a leg cramp and requires rescue by Yuuto, revealing vulnerability despite her martial arts training.

Across all media, including the *Purezza* season and OVAs, Shiina's role evolves from a potential romantic rival to a supportive friend. Her acceptance of Yuuto and Haruka's relationship demonstrates emotional maturity, though occasional melancholy over her unrequited feelings persists. She also appears in visual novel adaptations, with her role being more prominent in the anime.