Seto no Hanayome

During a summer visit to the Seto Inland Sea, Nagasumi Michishio nearly drowns and is rescued by a mermaid named Sun Seto. Seeing her true nature leaves them both in danger under mermaid law, which forbids humans from knowing their secret.

Marriage offers a way to spare their lives by making Nagasumi part of Sun's family. Unfortunately, that family is a yakuza clan, and her fiercely protective father has no intention of welcoming him. Keeping Sun's identity hidden becomes only one of the problems disrupting his formerly ordinary school life.