|Name in Japanese||三河 海|
|Rōmaji Translation||Mikawa Kai|
|Voice Actor (Japanese)||Daisuke Ono|
|Voice Actor (English)||Eric Vale|
Kai Mikawa (三河 海 Mikawa Kai ) is the richest boy in Japan, the fourteen-year-old son of the head of the multi-million dollar Mikawa Corporation. He rivals Nagasumi for Sun's hand in marriage, as they were once childhood friends. He is a killer whale mermaid from another mermaid group. Strangely while Sun and Lunar are shown in Mermaid form (torso of a human and tail of a fish) whenever they fail to maintain their human forms, Mikawa is shown as an entire killer whale when he succumbs to the effect of water.
He also has agoraphobia, which is why he travels only in submarines and wears an astronaut's suit in open spaces. The only place he doesn't wear the suit is inside the school. While looking for a servant he befriends Hideyoshi "Chimp" Sarutobi, who is the first person (besides Sun) to show him friendship. His name is related to Mikawa Bay and Neptune (海王星 kaiōsei ). Mikawa, like most merfolk, has a fear of cats (ailurophobia). He is usually seen in a Japanese navy uniform and carrying a katana.
While fixated on Sun, he is shown to be perverted as he enjoys looking at the bust of Akeno and enjoys the other girls' having moments.
- His family has a rule of being nice to women.