Isaki is one of two main characters, a merman who once claimed to not have a name. With meeting and moving in with Shima Kawauchi, he was named after the fish 'isaki', due to his connection of being a bit irritating alongside the actual isaki he owned in the freezer that had been growing old.

Isaki is shown to be a bit immature and innocent, despite the strong possibility that he is much older than Shima due to his claim of watching Shima when he was only a child. Alongside this, he shows a strong loyalty and attraction towards Shima.

Isaki's previous life in the ocean is said to be very sheltered and different than a human's life on land, as Isaki mentions that he has felt emotions and experienced things on land that he had never grasped beforehand.