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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. He is a killer whale 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. While looking for a servant he befriends Saru. His name is related to Mikawa Bay and Neptune (海王星, kaiōsei?).