Kai Ashimoto is a fourteen-year-old boy who serves as the central character in his story. He was originally from Tokyo, but his life changed course following his parents' divorce, which led him to relocate with his father to the small, sleepy fishing village of Hinashi Town. There, he lives with his father and his grandfather, who runs a traditional umbrella-making shop. This move represents a significant upheaval in his young life, as he is uprooted from the bustling city and brought to a quiet community where his family has a complicated history.

At the beginning of the narrative, Kai is portrayed as a withdrawn, gloomy, and pessimistic middle school student. He is often dismissive and blunt with others, struggling to express his complicated feelings about his family situation, particularly the absence of his mother, who remained in Tokyo to work as a dancer. His internal turmoil manifests as a lonely and apathetic demeanor, making it difficult for him to connect with his new surroundings or the people in them. The only real solace he finds is in creating music. He enjoys composing his own looped beats on a laptop and sharing them online, which serves as his private escape from the unhappiness he feels in his daily life. This musical talent is the key that begins to draw him out of his shell.

His role in the story is set in motion when his cheerful classmates, Kunio and Yuho, discover his online music videos and invite him to join their rock band. He initially meets their request with apathy but reluctantly agrees to practice with them on Merfolk Island, a local spot shrouded in legends about man-eating mermaids. It is there that he encounters Lu, a young, innocent mermaid. This meeting becomes the catalyst for his entire character development. Kai finds that Lu is not the monster of folklore but a friendly and magical creature who responds to his music. His initial curiosity about the local merfolk legend turns into a deep and unbreakable bond with Lu. He forms a tight-knit trio with Kunio and Yuho as they work together to protect Lu and hide her from the merfolk-fearing townspeople.

Kai’s key relationships are the driving force behind his emotional growth. His friendship with Lu is the most transformative, as her infectious joy and innocent desire for friendship directly challenge his pessimism. Their relationship is described as a positive, unbreakable bond reminiscent of a brother and sister, with Kai going out of his way to keep her safe from harm. He introduces Lu to the wonders of the surface world, and in turn, she helps him rediscover how to have fun and be honest about his feelings. He also navigates a strained dynamic with his father, who lost his passion for music, and his grandfather, who harbors a deep-seated grudge against merfolk after supposedly losing his mother to them. This family tension reinforces the town’s prejudice that Kai must learn to overcome.

Through these experiences, Kai undergoes a significant transformation. As he spends more time with Lu and his new friends, he begins to shed his withdrawn and gloomy personality. He becomes more fun-loving, cheerful, and positive about the future, learning to express what he is truly thinking and feeling. He learns to confront his own selfish moments, such as when he becomes disinterested in the band, and ultimately takes responsibility to help Lu and mend the rift between humans and merfolk in the town. In terms of abilities, Kai does not possess any supernatural powers. His most notable talent is his skill with music production, specifically creating digital beats and composing songs. This ability is the primary narrative device that attracts Lu to him and allows her to transform her fins into legs so she can walk and dance on land, making his creativity central to the plot's progression.