Mikeru

Mikeru is an ancient angel and is disgusted by what humans had done to the Earth that him and the other angels created. And starts a personal revenge against the Mermaid Princesses after learning their relations with the human world. Mikeru believes that angels and mermaids are superior beings to humans because both species understand the importance of the Earth and each have powers, in one form or another.

Although he is strong enough to resist any song the Mermaid Princesses would dish out against him until the finale, he is cursed with a weak body, so he steals both Kaito's memories and Seira's heart in order to keep himself alive.

Later on, it is revealed that the reason why his body is so fragile is because he and Mikaru are the same being, so both of them are "half" of what they should be during the most part of the season.

In the manga, it is revealed that Mikeru is under the control of Fuku and is very good and gentle at heart.