They seem to love their older sister, Ponyo, often schooling around her in the film, treating her like a leader. They speak few words, and appear to be very small children - almost newborn, although they may have lived a very long time. Ponyo surpasses them greatly in size and magical abilities. It is unknown what they truly are, or if they have names and magical abilities of their own.
Later in the film, due to Fujimoto's released potions, they too grow in size and number, fully transforming from girls resembling the size and hair length of Granmamare to immense fish made of ocean waves. They are reverted to their original forms after Granmamare appears to the women of the retirement home.
In the original Japanese version, Ponyo's sisters were voiced by Akiko Yano and Unknown in the English version.