Kai Mikawa, the 14-year-old only son of the Mikawa Corporation head, is the wealthiest boy in Japan and a merperson. Exposure to water triggers his unique transformation into a full killer whale, unlike other young merfolk who retain a human torso. His name references Mikawa Bay and Neptune. Severe agoraphobia, stemming from childhood trauma, compels him to wear an astronaut suit outdoors and travel exclusively by submarine; deprivation of these triggers panic. He also shares the common merfolk fear of cats. Kai typically wears a Japanese navy uniform and carries a katana.

He re-enters the narrative as a rival to Nagasumi Michishio for the hand of his childhood friend, Sun Seto. His fixation drives repeated attempts to undermine or eliminate Nagasumi, employing violent schemes and his vast wealth. Despite this rivalry, Kai collaborates with Nagasumi and Sun during critical confrontations, such as the final battle against Minamoto no Yoshio, where his submarine aids in breaching the antagonist's underwater palace. Kai exhibits perverse tendencies, including voyeuristic interest in female characters.

While searching for a servant, Kai befriends Hideyoshi "Saru" Sarutobi, who offers genuine friendship—a rarity for Kai outside Sun. This evolves into mutual loyalty, with Saru often assisting Kai despite eccentric demands. Kai's family adheres to a strict code of chivalry requiring kindness toward women, though his actions sometimes contradict this. His traits, including agoraphobia, are modeled after Shūtarō Mendō from *Urusei Yatsura*, with opposing phobias.

In later story arcs, Kai assumes leadership of Sun's bodyguard team. His wealth enables extravagant solutions, such as bribing individuals during agoraphobic episodes or deploying advanced technology. The OVA reveals his continued presence at Nagasumi's school and ongoing targeting by the Seto Group. While largely static, Kai displays moments of growth through his steadfast alliance with Saru and his pivotal role in the coalition against Yoshio.