Lucia is the Mermaid Princess of the North Pacific Ocean, keeper of the pink pearl, and the main character of the story. She goes up to land to search for her pearl which she used to save a boy seven years ago. She is unaware of the danger in the marine world and of her real mission until she is told of them by her "older sister", Nikora. She is characterized by her simplicity and naïveté, but with having a strong will, typical traits of shōjo manga heroines. Her idol form is Pink Pearl Voice. Of all the mermaids in the series, Lucia's mermaid form and human form contrast the most (Lucia has short dark blonde hair and brown eyes in the surface while having long lighter blonde hair and light blue eyes while at sea).
Hanon is the Mermaid Princess of the South Atlantic Ocean and keeper of the aqua pearl. Unlike Lucia, Hanon goes up to land to escape water creatures. She is perky and feminine, yet a bit more worldly than her friend Lucia. This worldliness is evidenced by many of her actions—especially by her romantic pursuit of her music teacher, Tarō Mitsuki. Her idol form is Mizuiro (Aqua) Pearl Voice, although the first time that it is named in the manga, she is erroneously called Blue Pearl Voice.
Rina is the Mermaid Princess of the North Atlantic Ocean and keeper of the green pearl. Initially she is depicted as beautiful but stern and bent on revenge for the capture of her best friend, Noel. Later, however, she evolves into a more mature character, and acts as an older sister figure to Lucia and Hanon. She seems to love plasma TVs and shows this love in many anime episodes. She also seems to love watching comedy shows. Her idol form is Green Pearl Voice.
Noel is the Mermaid Princess of the Arctic Ocean and keeper of the deep blue pearl. She helped Rina escape from the sea monsters, which led to her capture. She is calm, gentle, caring, and studious (as she sometimes wears glasses on land later in the second season of the anime). She is Caren's older twin sister by about a minute at the most. Noel cares deeply for Caren, the one that she had always wanted to meet. Her idol form is Ai-iro (Deep Blue) Pearl Voice.
Caren is the Mermaid Princess of the Antarctic Ocean, keeper of the purple pearl, and twin sister of Noel. In the anime, she first appears as the winner of a beauty contest that the main trio enters; in the manga, she first appears to save them from the Dark Lovers. In the first season, Tarō asks her about his mermaid, and she later blames Rina for Noel's kidnapping. Even after things are straightened out, she chooses to go find Noel alone. She is Noel's polar opposite, having a mysterious personality. Caren and Noel are twins, born on opposite sides of the Earth. When Caren finally meets Noel, she launches herself at Noel, hugging her and calling her Onee-sama, a respectful term for big sister. Even though the two had not met before, Caren cares most for Noel and would do anything to save her, even if it meant Caren having to find her on her own. While she is cold and distant in the first season, she is found to be wild, humorous, and even a bit strange in the second. Her idol form is Purple Pearl Voice.
Coco is the Mermaid Princess of the South Pacific Ocean and keeper of the yellow pearl. She is incarcerated by the sea monsters for most of the first season, so not much is known about her other than that she is a friend of fellow Mermaid Princess Sara. In the manga, she found to be playful and gentle, yet brash and outspoken in the anime. Her idol form is Yellow Pearl Voice.
Seira 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.
Sara is the Mermaid Princess of the Indian Ocean and keeper of the orange pearl. She joins Gackto in his quest to take over the marine world after an incident bestowed her a dark power, turning both her orange hair and her orange pearl pitch black (though her eyes stay orange) and destroying her own kingdom. That incident was a proposed rendezvous with Tarō, her secret human lover, in which he never showed up. (In the manga, he actually arrived before, but Sara's guardian found out about the relationship and insisted that he leave, convincing Sara that he had abandoned her.) In a fury, Sara unleashed a wave of power, destroying her kingdom and causing the wave which killed Kaito's parents and from which Lucia saved Kaito. In the manga, her villain name is Black Pearl Voice and she transforms into an idol in a black dress reminiscent of a diva. However, in the anime, she does not transform until she turns good, and instead has the power to sing offensively in a humanoid, amphibious form like the villains and mermaid idols, with or without her E-Pitch microphone. In the end, she decides to join the other mermaids as is her duty. She, herself, seals the gate to Gakuto's castle in the book because it's her duty and she unsealed the gate after her kingdom was destroyed. In the TV show, Aqua Regina seals the gate and Sara goes with Gaito because she loves him. The manga shows that she goes with him because she realizes that he is the one that needs her the most and that what he did was partly out of love for her and the want to make her happy. She also appears in the second season as a spirit, informing Lucia of the future Orange Pearl Voice and offering her assistance where she can. Sara is also seen singing with the other mermaid princesses in the very last episode of Mermaid Melody Pichi Pichi Pitch Pure.
Kaito, the son of a pair of musicians, is an avid surfer, as well as Lucia's love interest. Though he is popular with girls, he is secretly in love with a beautiful mermaid who saved him from a shipwreck seven years ago, which killed his parents, and has yet to realize that this mermaid is Lucia. Later in the story, it is discovered that beneath Kaito's human exterior lie some distinctively inhuman qualities and that he is the prince of an ancient water race called Panthalassa and is the younger twin brother of Gackto. Kaito has a hard time when Lucia comes in her human form, because of having feelings for both her human form and her mermaid form. Kaito eventually realizes that the two are one, giving Kaito the ability to love Lucia no matter what form she is in.
Lucia's guardian who serves as her guide. Hippo accompanies Lucia in her search for her pink pearl, and is among those who voice disapproval of her relationship with Kaito. He is also polite, always putting -さん, -san after every name, even when angry. In the anime, he is shown as Lucia's pet penguin. Later on, he is able to transform into a very cute human boy, as well as a creature called a Hippocampus. In his human form, Hippo falls in love with a Dark Lover, Yuri.