Ayuko Hayami is a former Olympic medalist in swimming who has become a strict and demanding swimming coach. Her athletic career is a defining aspect of her identity, and she brings the same intense discipline she once applied to her own training to the management of her swimming school. She has short black hair, brown eyes, and a tan, often appearing in a swimsuit that reflects her profession.

Her personality is initially presented as severe and unyielding. She runs her school with an iron fist, expecting her instructors to push students relentlessly and employing harsh methods, such as forcibly submerging children's heads to overcome their fear of the water. She believes that students are there to train, not to have fun, and that those who are afraid do not belong in the pool. However, this strict exterior masks a deep-seated love for swimming itself, which originated from joyful childhood memories of swimming with her father. Her true motivation is a desire for her students to love the water as much as she does, even if her methods for achieving this are initially misguided.

In the story, Ayuko serves as the primary focus of the fourth episode. Kintaro Oe, the protagonist, becomes enamored upon seeing her, calling her "The Beast," and takes a job at her school to be near her. After he fails to meet her standards, he challenges her to a swimming competition. While Kintaro focuses on coaching the beginner classes with patience and encouragement, Ayuko observes his progress and her own students' positive responses to his methods. Her development is catalyzed by this experience; she comes to realize that the love and joy her father taught her, rather than harsh discipline, were the true keys to her own success. Consequently, she recants her severe training philosophy and hopes for Kintaro's return, though she had previously fired him for an inappropriate incident at the pool.

Key relationships are central to her character. Her interaction with Kintaro is a primary driver of her change, as his unconventional and kind-hearted teaching style contrasts sharply with her own, forcing her to re-evaluate her approach. She develops a romantic interest in him, and in the final episode, she joins the other women from the series in chasing after him. Her notable ability is her elite-level swimming proficiency, a skill she puts to use when she accepts Kintaro's challenge. She reappears briefly in the sixth episode, where she is asked to model for a difficult swimming animation sequence, demonstrating that her athletic form is still a subject of admiration.