Kaito Domoto is the teenage male lead and the primary love interest in the story. He is introduced as a handsome, popular, and highly skilled surfer, known as the champion of the East Coast, which attracts many admirers at his school. His background reveals that he is the son of a famous pianist and his wife, both of whom tragically perished in a shipwreck when Kaito was young. Kaito himself was saved from drowning in that same incident by a mermaid, an event that shapes his entire character arc.

In terms of personality, Kaito is typically portrayed as kindhearted, loyal, and helpful, though he can also be snarky and cool-minded. He is dedicated and serious about his passion for surfing, which is his greatest strength. Despite his popularity, he remains devoted to the memory of the mermaid who saved him seven years prior, which causes a central conflict as he is simultaneously drawn to the human girl Lucia Nanami, not realizing they are the same person. He is often shown to be understanding and patient, frequently needing to explain situations to the sometimes jealous Lucia to clear up her misunderstandings.

Kaito's primary motivation for much of the first story arc is to find the mermaid who rescued him. He carries a deep, unshakable affection for her, keeping the pink pearl she left with him as a treasured keepsake. His role in the story is multifaceted. He is Lucia's main love interest and a central figure in her life on land, but he also becomes a key protector. When Lucia and the other mermaid princesses are in danger, a mysterious power awakens within Kaito, marked by a glowing trident symbol on his forehead, which allows him to combat the powers of their enemies.

His relationships are crucial to the plot. His bond with Lucia Nanami is the emotional core of the series, beginning as a friendship where he is unaware of her true identity and eventually developing into a loving and healthy romantic relationship built on mutual support and loyalty. A pivotal relationship is with Gaito, who is revealed to be his twin brother. This revelation uncovers Kaito's true heritage; he is not a normal human but a prince of the Panthalassa, an ancient water race, making him the younger twin brother of the primary antagonist of the first season. Their relationship is marked by public conflict due to their opposing sides in a war, but underneath the bickering, Kaito genuinely cares for his brother and regrets their inability to live in peace.

Throughout the story, Kaito undergoes significant development. He starts as a boy searching for a phantom memory and transitions into a young man who accepts the merging of his two loves. He eventually realizes that Lucia and his mermaid are the same person, which allows him to fully commit to her. In the second season, Pure, his development takes a challenging turn when he moves to Hawaii for a surfing competition, suffers a severe accident, and falls into a coma. Upon being rescued, he loses his memory, particularly of Lucia and the other mermaids. His arc then focuses on recovery and regaining his lost memories with Lucia's help.

Kaito possesses several notable abilities. Beyond his exceptional talent for surfing, which he uses both as a skill and a pastime, he holds the pink pearl that originally belonged to Lucia. This pearl creates a powerful resonance between them, allowing him to enhance the mermaid princesses' singing powers, which are their primary weapon. He also demonstrates a unique connection to marine life, showing an ability to understand the feelings of sea animals. His most dramatic ability is the latent power inherited from his Panthalassa lineage, which manifests as a protective force that can repel the attacks of water demons when Lucia is in peril.