Momo-chan (ももちゃん?)
Voiced by: Naomi WakabayashiA pink dolphin from Lucia's realm. Initially appeared being showcased in an indoor aquarium for curiosity. After being rescued by the Mermaid Princesses and reunited with its mother, its role is later reduced to occasional messenger. While the Mermaid Princesses could communicate with it using their inner voices, whenever it is with Kaito, he can easily understand the meaning of its gestures.
Nagisa Shirai (白井 渚 Shirai Nagisa?)
Voiced by: Chihiro KusakaNagisa is a young boy who has a crush on Hanon. After retrieving some sheet music for Hanon, he has been wooing her ever since. Unfortunately for him, the sheet music is of the tune Tarō Mitsuki dedicated to Hanon, which only reminds her of her fixation on the much more refined and older man. Still, his resilience is to be admired, and she eventually treats him better. In the manga, he says that he is two years younger than Hanon. In the end, Hanon falls in love with Nagisa and they have a relationship at the end of the manga.
Masahiro Hamasaki (浜崎雅弘 Hamasaki Masahiro?)
Voiced by: Kiyotaka FurushimaMasahiro is a flirtatious boy who has a crush on Rina. He wears glasses, rides a motorcycle and does some amateur boxing. It is later revealed that he comes from a family running a boxing gym. At the end of the manga, Masahiro is Rina's boyfriend.
Rihito Amagi (天城リヒト Amagi Rihito?)
Voiced by: Takahiro MizushimaRihito is an orchestra conductor and Michal's older brother. After initially seeing Kaito and Lucia at the airport, he becomes a "bridge" for the two. He observes Kaito's progress in regaining his memories (despite his sister being around) while he checks on Lucia and comforts her as she copes with the loss of her emotional bond. Rihito tries hard to get Michal to realize that she could never be with Kaito even though she doesn't want to hear it. In the anime, when Kaito leaves after telling Michal that he loves Lucia, Rihito restrains Michal from going after Kaito. When Michal is reborn, he promises that he will care for her and raise her again. It is later revealed that both he and Michal are part of the same race as Kaito, leaving Rihito to believe that that is the true reason why Michal is attracted to Kaito.
Michal Amagi (天城みかる Amagi Mikaru?)
Voiced by: Ryoko ShintaniMichal is the girl who finds Kaito after he had lost his memories. She's a rich, yet desperately lonely girl who has lost all hope; she falls "in love" with Kaito to the point that, in Lucia's perspective, he has forgotten his mermaid love completely, and is desperate that he return her feelings. Michal also seems to be a sickly girl, collapsing to the floor every time Michel appears somewhere else or when Kaito is near Lucia (in order to keep Kaito to herself). In the manga, when Kaito states that he loves only Lucia and could never love Michal the way that she "loves him", Michal takes off her clothes and offers herself to him in desperation of keeping him; Kaito turns away from her and leaves. This is a large trigger for her collapses and allows Michel to take control of her. In the end, Michal is reborn and given to her brother to raise her from scratch.
Seira (星羅?)
Voiced by: Eri KitamuraSeira is the future Mermaid Princess of the Indian Ocean and future keeper of the orange pearl. She appears as a spirit to Lucia, taking the form of a young mermaid. When her time of birth finally came, Michel seized his chance and absorbed Seira into his body, and her birth was therefore delayed until Lucia could retrieve all the pieces of her heart contained in Michel's feathers. Every time Lucia retrieves a piece of her heart from one of Michel's fallen feathers, Seira learns something about an emotion, such as love or friendship. Once born, she is destined to become Orange Pearl Voice. Seira is very child-like and sees the good in Michel, realizing that he has good intentions for the earth.
Aqua Regina (アクア・レジーナ様 Akua Rejīna-sama?)
Voiced by: Kumi YamakadoAqua Regina is the ocean goddess who gives the Mermaid Princesses new songs and upgrades for their gear. However, this is all that she can do alone after expending a great deal of energy thousands of years ago. She is needed to ultimately stop the sea monsters, and the seven pearls have to be gathered to summon her; Gaito/Gakuto seeks to summon her as well, so that he can use her to destroy the world. Aqua Regina later gives bands to the mermaids to upgrade their singing power; as in many magical girl series, this is signified by adding frills and accessories to their dresses. She also upgrades their microphones in the end of season one, before she is finally summoned. At the end of the manga, Aqua Regina reveals that Lucia is to be her successor.
