These are the first antagonist to show up in the series. Led by Gaito, they aim to take over the sea world and rule it using the power of the seven mermaid pearls.
Gaito is the leader of the sea monsters who is bent on taking over the marine world. He is always depicted drinking wine, and wearing black. His resemblance to Kaito made the latter a subject of initial ire by Rina. He is known in the anime as Gaito. He is in one episode turned into Kaito and seems to get feelings for Lucia's human form. He was lonely half his life at the bottom of the sea until he met Sara the orange pearl princess. He loved Sara, and always tried to please her with "gifts".
The Dark Lovers
Gaito's four female followers who serve him in exchange for his love for them. There are composed of the following:
Izuru is the most mature of the four, she has the power to control waves and marine beasts and has the ability to create dragons from water. She is also the most obsessed with winning the heart of Gaito.
Eriru is the one with the multiple personality disorder. At one phase, she is cute, beady-eyed, and acting as if she were antsy (spinning around and never saying any normal speech). In another phase she can speak correctly and is more observant of her surroundings. In her third phase, when threatened (or when she spins around), she becomes scary, her eyes glow red, and she changes her voice accordingly.
Maria is the one closest to Gaito in terms of love. She seems to be the most innocent of the group, but in actuality she can be very cruel at times, and sometimes seems to see Gaito more as a boy-toy than a love prospect. She has the power to make and/or manipulate ice and snow.
Yūri is the youngest-looking of the four. She has the ability to control people and objects, especially with the use of her music and, if any, a nearby piano. Although Yūri is loyal to Gaito, his continued treatment of her as a child leads her to a star-crossed relationship with Hippo, i.e. his human form.
Black Beauty Sisters
They are composed of the red-wearing older sister Sheshe and the blue-wearing younger sister Mimi. Gaito hires them after the Dark Lovers' failed attempts to capture the Mermaid Princesses. These two have the ability to sing tunes more powerful than the songs the Mermaid Princesses themselves sing, although in the anime and the second arc of the manga, the Princesses manage to get strong enough to overpower them. Though Gaito turned them back to their original forms (two angler fishes), they are brought back in the second season after being revived by that season's antagonist, Michel, and made them his servants. It has been shown that Mimi has a human form. She is the only one whose hair gets longer and wears glasses over her (in human form only) brown eyes. In human form, Sheshe looks the same, but loses her horns, demon ears, and other inhuman body parts.
Sara is the Mermaid Princess of the Indian and keeper of the orange pearl. She joins Gaito 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 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 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. 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. The pictures below are of her when she isn't evil and in her original color.