* Rin. The protagonist of the novel, Rin is a young girl who is sold to a travelling circus. She is later rescued by the Prince of the Mist, a mysterious figure who takes her to his kingdom under the sea.
* The Prince of the Mist. A powerful and enigmatic figure, the Prince of the Mist is the ruler of the underwater kingdom. He is both kind and cruel, and Rin is unsure whether she can trust him.
* Siora. A mermaid princess, Siora is Rin's friend and ally. She is kind and compassionate, and she helps Rin to learn more about the underwater kingdom.
* Neel. A young merman, Neel is Siora's brother. He is impulsive and reckless, but he is also loyal to Rin and Siora.
* Elara. A powerful sea witch, Elara is the Prince of the Mist's sister. She is cruel and ambitious, and she wants to take over the kingdom.
* The King of the Sea. The father of the Prince of the Mist and Elara, the King of the Sea is a wise and powerful ruler. He is also very protective of his children.
* The Great Squid. A giant squid that lives in the depths of the ocean, the Great Squid is a fearsome creature. It is also very intelligent, and it is sometimes able to communicate with humans.