|Japanese name:||瀬戸 燦|
Weight: 45 kg
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.
Voiced by: Haruko Momoi (Japanese), Alexis Tipton (English)