Yonka

The Medaka Mermaid Quintuplets (メダカの人魚の五つ子 Medaka no Ningyo no Itsutsu-go) are five tiny young mermaids who are friends with Keimi. Their names are Ichika, Nika, Sanka, Yonka, and Yonka Two.

Yonka has pink hair and a brown-striped tail, her top is brown and her hat is white.

The five sisters tend to talk in a unique speech pattern. Ichika starts off first by saying a regular statement. Nika then states an exclamatory version of Ichika's statement. Sanka then states the opposite of the original statement. Yonka then questions the original statement. Lastly, Yonka Two then questions whether the original statement matters or not. An example of a typical routine:
-Ichika: Luffy is a pirate.
-Nika: Luffy is a pirate!
-Sanka: Luffy isn't a pirate.
-Yonka: Luffy might be a pirate.
-Yonka Two: So what if Luffy's a pirate?

Other than that, they are cheerful and friendly and they usually stay close together.

First Appearance: Chapter 608; Episode 527

(source: One Piece Wikia)