Yuiichi is Enina's love interest. He is interested in surfing and wants to be a professional surfer one day. But he cannot surf because he is too young. He is nine years old. He is just a human, but he does not know that Enina is a mermaid princess. He sort of resembles a nine-year-old Kaito Domoto. It is revealed that he is also in love with Enina. Luna tells Enina that Yuiichi will marry her and become an ocean prince. Enina is soon aware of this after a couple minutes of disbelief. Yuiichi kisses her for the first time and proposes going steady. Enina says yes and they are happily a couple. He feels very proud of himself when he finds out that Enina is a mermaid. But she tells him not to brag about it to his friends and he disagrees. They get into a fight for while but soon make up. In a future manga of the mermaid princesses as adults, they get engaged. Yuiichi finally becomes a surfer and starts his own TV show for kids who want to surf. Enina becomes a fashion designer just like Hiroko and makes her proud. They have a baby girl and name her Samako. Yuiichi has a long history ahead of him.