Yuuko Yoshikawa is a violin-playing student at Siren Academy, a prestigious girls' boarding school. She belongs to the group discovering supernatural occurrences at the academy, including a goblin-like creature appearing in a photograph taken during the new semester's opening ceremony. When the student body succumbs to profound lethargy, Yuuko's violin playing briefly revives her peers, showcasing both her musical skill and concern. Yet this revival proves temporary, and the underlying apathy endures.

Her background includes interactions with fellow students Atsuko Yoshikawa and Sayaka Matsumoto. Yuuko is present during critical events, such as Professor Goda's attack by a fairy-like entity named Rom within the school church while he listened to her violin performance. This incident directly links her to the emerging supernatural threats at Siren Academy. Her role centers on these initial supernatural discoveries and her momentary alleviation of the students' lethargy through music.