Hailing from Otaru, Hokkaido, Shiina Amamiya transfers into Hakujou Academy, joining Yūto Ayase and Haruka Nogizaka's class while avoiding discussions of her past. Her relocation is partly driven by the school's famed "Performer of the Sunset" instructor. Shiina possesses exceptional piano skills, securing second place at the London Piano Contest—where Haruka took first—and practices naginatajutsu with notable proficiency. Her appearance features purple hair, violet eyes, and a 1.60-meter stature.

Naturally cheerful, sociable, and positive, Shiina adapts smoothly to her new environment. She handles situations with directness, confidence, and proactivity but turns shy regarding romance. Repeated chance encounters with Yūto ignite her romantic feelings, which she interprets as fateful signs, sparking internal conflict over confessing. Their interactions sometimes lead to compromising scenarios like accidental panty exposures or suggestive positions, demanding hasty "it’s not what it looks like" explanations. When a leg cramp strikes during the swimming tournament, Yūto rescues her, intensifying her attachment.

For the cultural festival, Shiina organizes her class’s cosplay café and recruits Yūto to help. A misunderstanding arises after Haruka discovers them in an ambiguous pose, straining Haruka and Yūto’s relationship temporarily. Shiina’s arc culminates during the Hokkaido class trip (*Finale*), where she confesses to Yūto. Haruka overhears, causing distress, but Yūto rejects Shiina, affirming his commitment to Haruka. Shiina accepts this gracefully, prioritizing their happiness over her unrequited feelings without sabotaging their bond.

Throughout these events, Shiina maintains close friendships with classmates Mai Asahina and Ryōko Sawamura. Both recognize her affection for Yūto and encourage her pursuit, yet Shiina remains indecisive and occasionally aids in bringing Yūto and Haruka closer.