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.!~ 

After the final battle against Gaito, Sara had given her life to remain with him. The sea world is now at peace, but only with six mermaid princesses. 
While Caren, Noel, and Coco have returned to their kingdoms, it should be time for Lucia, Hanon, and Rina to do the same. After all, Mitsuki Tarou is leaving for Germany, and Doumoto Kaito is heading to Hawaii. Lucia is devestated, and when things couldn't get any worse, Kaito is lost at sea! Worst of all, a strange angel named Michel has appeared, wanting the mermaids to join him. But on the lighter side, Sara's spirit tells Lucia of Sara's successor, a young mermaid named Seira. The mermaids are in for another adventure!


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Lucia is the new girl at school. She and her sister run a public bath that's all the rage. When Lucia meets a terrific-looking surfer boy, there's just one little problem: Lucia is a mermaid-not just any mermaid, but a princess on an important mission to save the seven seas from an evil force bent on taking control of the marine world. Such a responsibility doesn't leave much time for romance. But Lucia vows to protect her world and win the heart of handsome Kaito. 
(Source: Del Rey) 
Note: Includes the one-shots Moonlight Goddess Diana (MOONLIGHT GODDESS ディアナ), Nude ni Natte (ヌードになって, Get Nude) and Cherry Blossom (チェリー ブロッサム), contained within Volume 7 of the manga.