Gou Matsuoka

The heroine of this work. Manager of the Iwatobi High School Swim Club. In the first season, she is a first-year student, in the second season, a second-year student, and in the third season, a third-year student. She is Rin's younger sister by one year. She is 160 cm tall, but her weight is a secret. She admires her brother Rin, and after Rin returned to Japan and moved into the dormitory, she was worried about him not coming home and barely contacting her. This concern led her to get involved with the swim club along with Haruka and others, who were old friends of Rin through swimming.

Although her name is the same as Gou (江) Asai Nagamasa's third daughter, it is not old-fashioned or modern, and she disliked the masculine sound of her name, often insisting on being called "Kou." However, she eventually accepted being called "Gou" by those around her. She has a preference for well-built male bodies and is knowledgeable about muscle parts and names. She can recognize Makoto just by looking at his leg muscles.

She joins the swim club as the fourth member and serves as the manager. She actively finds practice locations and opponents before the athletes even ask, collects and analyzes data to set schedules and training camps, and engages in various activities.