Mamiya is a supporting character in The Dangers in My Heart: The Movie, a compilation film that recounts the events of the first two seasons of the anime. She is a third-year middle school student with short brown hair and brown eyes. Mamiya is introduced as the childhood friend of Haruya Nanjo, and her primary motivation is her unreciprocated romantic feelings for him. She secretly likes Nanjo and wishes to advance their relationship beyond friendship, but she struggles to capture his attention. This dynamic is complicated by Nanjo's interest in Anna Yamada, which causes Mamiya to experience jealousy and frustration. Her role in the story is to add a layer of minor romantic tension and social complexity within the broader school setting. While she is not a main character, her interactions primarily revolve around Nanjo, and she occasionally appears in group scenes that highlight her hidden infatuation and her efforts to navigate her feelings. Mamiya does not undergo significant development in the film, but her presence underscores the theme of unrequited affection among the supporting cast. She is a typical student with no exceptional abilities, and her character serves to illustrate the quieter, less central romantic subplots that exist alongside the main relationship between Kyotaro Ichikawa and Anna Yamada.