Kai is the main protagonist of this animated feature. He is a middle school student who has moved with his father to the remote fishing village of Hinashi-cho following his parents' divorce. His mother, a dancer, remained in Tokyo, while his father, a former musician, now runs the family umbrella shop, having seemingly lost his passion for music. This family upheaval has left Kai feeling isolated and withdrawn. At school, his days are filled with a sense of gloom, and he finds it difficult to express his complicated feelings to those around him. His only emotional outlet is writing songs and uploading them to the internet, a secret creative life he keeps to himself.

Initially, Kai is characterized as being dismissive, blunt, and emotionally cut off. He is depicted as a melancholy and pessimistic boy who has trouble connecting with his new surroundings and his father’s expectations that he should lead a stable, secure life. However, his reluctant circumstances change when his lively classmates, Kunio and Yuho, invite him to join their band, SEIREN. He follows them to their practice space on Merfolk Island, where he meets Lu, an exuberant young mermaid. This encounter is the catalyst for his transformation. As he begins spending time with Lu, whose innocent singing and dancing are infectious, Kai gradually emerges from his shell. He becomes more fun-loving and cheerful, learning to express his true feelings and rediscovering a sense of joy and positivity about the future.

Kai's role in the story is that of a bridge between two worlds: the human world and the world of the merfolk. His personal journey of overcoming his internal struggles is directly tied to the central conflict of the film. Through his bond with Lu, he inadvertently exposes the secret of the merpeople to the townspeople, reigniting an ancient feud rooted in fear and superstition. When a crisis arises that threatens Lu and floods the town, Kai’s emotional growth is put to the test. He takes a stand to protect Lu, help his friends, and resolve the conflict, ultimately playing a key role in forging a new bond of friendship between the humans and merfolk. His musical talent is also a key plot point, as he is secretly composing a song throughout the movie, which he only gains the confidence to perform publicly at the climax, using it as a final expression of his newfound emotional openness.

His most significant relationship is with Lu. The two form an unbreakable platonic bond, with Kai going out of his way to protect her from danger. He is first drawn to her because she reminds him of his mother, and her pure spirit inspires him to express himself again. His other key relationships are with his bandmates, Kunio and Yuho, who are instrumental in pulling him out of his isolation and providing the social framework for his adventures with Lu. Developmentally, Kai transforms from a silent, withdrawn boy who keeps his feelings bottled up into a proactive individual capable of love, friendship, and artistic expression. He is a talented musician and songwriter, which serves as his primary means of processing his emotions. He also seems to have an academic interest in merfolk, as he is shown reading books about them before he ever encounters Lu.