Melmo was given a bottle of candy from God. The blue candy turns Melmo into a lovely 19-year-old version of herself, while the red candy turns her back into a child. Combining the two turns her first into a fetus, then into an animal of her choosing. When Melmo ages from 9 to 19 her clothes don't grow with her, usually resulting either in shredded clothes, or skin-tight children's clothes on the body of an adult woman.

Melmo is a kind-hearted young girl who is frequently torn between her actual age and the things she encounters and wants as her older self.

The 19-year-old Melmo is quite beautiful and men frequently fall in love with her.

In the last few episodes of the series Melmo's 19-year-old self acquires a boyfriend, Jiro, who she eventually marries. During the final episode - and several years after Melmo's and Jiro's wedding - Melmo gives birth to a baby girl, in whom resides her late mother's spirit, thus reuniting mother and daughter (and, interestingly, reversing their relationship with each other in the process).

Melmo as a 19-year-old is no smarter or more experienced than her 9-year-old self, though at 19 she is given more opportunities to learn about adult matters than she would at the age of nine.
(Source: Wikipedia)