Marli is the Swordfish Princess, with her crown being a cheerful swordfish hooked in her hair. In the cartoons, she is shown as an arrogant girl and a rival to Ester, wanting to overcome her at school and at any activity. However, she is a good girl despite all this.

In the books her personality is different compared to her depiction in the cartoons. She is a very intelligent girl, who can work without a lot of effort. However, Marli is very shy, tending to listen to everyone's opinions before giving her own, often covered with reason. As a result, her friends think that she does not pay a lot of attention.

Marli is 7 years old and has the sign of Pisces.