Otoha Biwakōji is a main character who studies at Shōnan Music Academy, a music-focused school in the same coastal region as the rest of the cast. She enters the story through her friendship with Ayame Nakahara, the radio enthusiast who lends technical expertise to the group of girls working to revive an abandoned mini FM station called Aquamarine. When Ayame joins the project, she brings Otoha along specifically to handle the musical side of the broadcasts, and from that point Otoha becomes the primary creative musical contributor to the station. She writes and produces several jingles and original pieces of music for the revived radio programming, and Ayame introduces her on air as a current high school student and an outstanding singer-songwriter. Her role in the story is that of a gifted collaborator who elevates the group's amateur broadcasts with accomplished musical work, while remaining somewhat less involved than the others in the day-to-day running of the station and in the personal conflicts that arise around it. Her most notable abilities are musical in nature: she composes, performs, and produces songs and short jingles that give the station a polished and recognizable sound, and her skills are clearly respected by the others. Her key relationship is with Ayame, who acts as the bridge between Otoha and the wider group; through Ayame she also comes to work alongside Nagisa, Shion, Kaede, Shizuku, and Yū, particularly when all of them come together for the final broadcast. Because the story unfolds over a single summer, Otoha does not undergo a major personal transformation on screen; her development is instead shown through her deepening involvement in a shared project and her steady, dependable support of her friends. The film provides only limited information about her personal background, and her inner life and private struggles are not explored in depth. In the novelization of the story, she later moves into the music world and performs live, which fits the impression left by the film that music is the center of her life and her main way of contributing to the people around her.