These are the mermaid princesses of the ruling seas right now: the North Pacific Ocean, the South Pacific Ocean, the North Atlantic Ocean, the South Atlantic Ocean, the Arctic Ocean, the Antarctic Ocean, and finally, the Indian Ocean. Each princess holds on a pearl which is the source of their power and song. When combined, they can do many great things.
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. It turns out that the boy she saved is Kaito. But upon realizing she can't straight on tell him she is a mermaid (since she'll turn to foam is she does), she tries everything she could to get Kaito to realize that she is the same mermaid the saved him those seven years ago. Because her kingdom in the North Pacific Ocean is one of the last kingdoms in the ocean that hasn't been attacked by Gaito, she is unaware of the danger in the marine world and of her real mission of gathering together the other six Mermaid Princesses and summoning the sea goddess Aqua Regina to save them until she is told of them by her "older sister", Nikora. She is characterized by her simplicity and naïveté, but has a really strong will when she wants to get something done. 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. She is Lucia's best friend, but fights with her like any teenage girl. Unlike Lucia, Hanon goes up to land to escape water demons that had attacked and destroyed her kingdom in the South Atlantic Ocean. After meeting up with Lucia and finding out that she is also a Mermaid Princess, she moves into the Pier hotel that Lucia is staying and working in with Nikora. She is perky, feminine and a bit more worldly than Lucia (even giving her dating advices in case Lucia and Kaito ends up in a situation alone together), although she tends to be lazy in cleaning, especially when it came to helping Lucia clean the hotel. This worldliness is evidenced by many of her actions—especially by her romantic pursuit of her music teacher, Tarō Mitsuki. Her idol form is 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. Her kingdom in the North Atlantic Ocean, like the other kingdoms of the sea, has been destroyed by Gaito and she alone had escaped capture. Initially she is depicted as beautiful but stern and bent on revenge for the capture of her best friend, Noel who tried to save her after the fall of both their kingdoms. She even mistakes Kaito for Gaito at first sight as was really cold to him. 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 (including reffering to Caren as the "girl who won the plasma tv" for a while before learning her name". She soon meets Masahiro, who she started liking and begins to date by the end of the Pure season of the series. 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 after the fall of both their kingdoms, 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. Legend has it that the minute these twins were born, they were separated from each other to their respective poles of the Earth, to their new kingdoms. They have never met each other for their entire lives until Caren goes to save Noel from Gaito's castle with the other Mermaid Princesses. 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, after she becomes better friends with Lucia, Hanon, Rina, and the rest of the Mermaid Princesses. 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. She is 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. Though she is never mentioned until the Pure season of the series, 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.