Momo is a princess of "Fenarinarsa" (フェナリナーサ Fenarināsa), "the land of dreams in the sky". Fenarinarsa is the dwelling place for fairy tale characters. But it was in danger of leaving Earth's orbit and disappearing, because people in the earth lost their dreams and hopes. The king and queen of Fenarinarsa sent their daughter Momo to Earth to help them regain their dreams. Momo became the daughter of a young childless couple in the earth, accompanied by three followers with the appearance of a dog (Sindbook), a monkey (Mocha) and a bird (Pipil). On Earth, Momo takes the appearance of a teenaged girl. To help the planet regain its hopes and dreams, Momo transforms into an adult form of herself, with an occupation tailored to fit the situation. Each time Momo succeeds in bringing happiness to the person affected, a jewel appears in the Fenarinarsa crown. Once 12 jewels appear, Fenarinarsa will return to Earth's surface.

(From: Wikipedia)