Black Beauty Sisters

The Black Beauty Sisters are 2 sisters that work for Gaito and in the second season, they work for Mikeru. They sing dark songs to fight against the mermaid princesses. They have 2 songs called Kuro no Kyosokyoku (Voice in the Dark) and Yami no Baroque (Dark Baroque). Throughout the series they appear to act more then just sisters, more like teasing rather then a relationship.

SheShe
Sheshe is the elder sister of the Black Beauty Sisters. Sheshe is a mercenary of Gaito, not in love with Gaito like the Dark Lovers, but only willing to cooperate with Gaito for personal gain (more specifically, a piece of the 7 oceans). The Black Beauty Sisters are the most powerful servants of Gaito, with a singing voice so strong that the mermaids can barely stand to listen to it. The reason the Black Beauty Sisters must align with Gaito is because they, like the Dark Lovers, are also transformed by Gaito's spell. Sheshe's true form is that of a demon anglerfish.

Mimi
Mimi is the younger sister of the Black Beauty Sisters. Mimi is a mercenary of Gaito, and not in love with Gaito like the Dark Lovers. Mimi is more of a follower of Sheshe, and in their song, she sings the chorus. Their song causes pain to mermaids. Like her sister, Mimi is also transformed by Gaito's spell, and her true form is a blue demon anglerfish.