"What were you thinking about in the pool's sanctuary?!"~Ayuko to Kintaro.
Appearing in Episode 4, Ayuko Hayami (often mistranslated as Ayuko Hayamizu) is a former Olympic swimming medalist that now works as a swimming coach. Upon first seeing her, Kintaro is immediately enamored and describes her as "The Beast": an untamed wilderness that was able to awaken the "beast" in his heart. He applies to be a swimming coach at her school, but after failing to meet even adequate swimming standards, Kintaro challenges her to a swimming competition in a month's time of training. If he wins, Ayuko must accept him as a swimming coach. But if she wins, Kintaro is forced to do her bidding for an entire year.
Instead of spending most of his time practicing swimming himself, Kintaro instructs the first three beginner classes, where he constant had to deal with scared children, vomiting, bathroom issues, and more. Nonetheless, he doesn't give up on his students and makes slow progress at the pace they are comfortable with. Opposite of Kintaro, Ayuko runs her swimming school with an iron fist: her instructors regularly fail students who don't meet her strict demands and resort to questionable methods, such as forcibly submerging children's heads. After a few week's time of voluntarily helping Kintaro train for their race, she is surprised at how much progress he made in such a short span of time. During a practice swim, Kintaro unknowingly awakens "the beast within him" while swimming behind Ayuko, swimming so fast that he catches up to her and ends up having his face scissored between her legs. Irritated with the desecration of sanctity of her pool, she punches Kintaro in the face, knocking him out. A few hours later, upon regaining consiousness, Kintaro finds himself half-naked on the lawn outside of the pool with his Cresent Moon and other belongings. He started having a cold from the cold air and feels ashamed about what he did. Getting back on his feet, Kintaro get his clothes back on and sets off yet again, thinking he'll come back when things calm down.
Days pass following Kintaro's sudden absence from the pool, yet his impact on both the children and other staff did not. The children in the beginner classes had learned so much that many more students were advancing into higher levels than before. This was the result of the staff slightly adjusting their curriculum by using Kintaro's example of instruction to help their students learn (for instance, they have the children try to get a submerged ring using their hands, having them submerge their heads on their own rather than their previous "iron fist" methods). While one parent was thanking Ayuko for the progress that her child was showing, the child even asked where "Coach Kintaro" was. After realizing how fun yet effective Kintaro's introduced teaching methods were with the children, Ayuko believes it should be the crux of her future swimming instructions, reminding her of the times she went swimming with her father (which made her want to become an Olympic swimmer). Ultimately, she recants her harsh training methods. Since she had already fired Kintaro and threw him out, Ayuko can only hope that he might return, where she would gladly welcome him as an official swimming coach.
She makes another appearance in Episode 6, when Kintaro asks for some recording of some swimming as a model for a film at an animation firm he temporarily works at.