The female protagonist of the story, Sun is the daughter of the Yakuza Seto Group's boss, Gōzaburō Seto. She is a firm believer in the chivalrous spirit of mermaids, and is devoted to her role as Nagasumi's wife. She is also in love with Nagasumi. Since she has yet to become an adult, Sun is unable to completely maintain her human form, and her legs revert to a tail whenever she comes into contact with water. She often cites the similarity between the Japanese words "ninkyō" (chivalry) and "ningyo" (mermaid) to stress her adherence to honorable courses of action; in the Funimation release of the anime, her assertion is rephrased as "honor amongst thieves is honor under the seas". When Sun enters chivalry mode cherry blossom petals can be seen in the background. It is later revealed during Lunar and Nagasumi's wedding that if the need arises she can fight very well with a sword and also shows behavior much like a Yakuza member. Her name is related to Seto Inland Sea and is a direct reference to the Sun. She has her own personal body guards but does not use them same way that Lunar does. They only show up on matters involving Lunar’s personal body guards whom they (Sun's body guards) are constantly at war with. Like most mermaids, she has a fear of cats (ailurophobia), but is capable of getting over it on occasion, like when the kitten that lives in Nagasumi's house is in danger of drowning, and she goes to save it.

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