Ponyo, full name Brunhilde, is a 5 year old girl, and the main protagonist in the movie, Ponyo on the Cliff by the Sea.
Originally born in the sea, Ponyo lived with her many sisters and her father, Fujimoto, a human scientist who lives underwater. She met Sosuke, a human, after wanting to see more of the world while floating on a jelly fish. She licks Sosuke's finger, which has blood on it.
Later, Ponyo and Fujimoto get into a fight, for Ponyo wishes to be human, and denounces her original birth name, Brunhilde. Because of the human blood she ingested, she takes the body of a human, and goes to Sosuke, and his mother Lisa.
Ponyo causes the world to become out of balance, thus starting a tsunami and making the moon, among other things, out of place. But, if Sosuke, who must pass a test to let Ponyo be human, fails, she will turn into sea foam, and the world will be restored.
Before finding Lisa, in the nursing home after it sank, but herself and the residents were giving temporary the power to breathe underwater, Ponyo turns back to her fish form.
Granmamere, otherwise known as the Goddess of Mercy, asks Ponyo if she chooses to be fish or human, which Ponyo chooses human. At the end of the movie, Sosuke kisses the bubble in which Ponyo is in, thus turning Ponyo to human form, and removing her of her powers. The balance of nature is restored at the end of the movie.