The Antarctic Ocean Mermaids live in the Antarctic Ocean Castle. Their princess and ruler is Karen. The Mermaid's appearance are in different shades of purple. They only appear in a flashback of Karen in Episode 27 where the mermaids informed Karen that her twin sister Noel was captured by Gaito.
List of Antarctic Ocean Mermaids
The Antarctic Ocean mermaids need to talk with mermaids from other kingdoms. To do so they use penguins.