The Frog Princess is a frog that is transformed into a human, after the player kisses her to break the spell, during the Kiss the frog random event. If the player is a female, however, the frog turns into a Frog prince instead.
A player kissing the frog to transform her back into a Princess
According to the Princess, she had been transformed into a frog by a wizard, who has an obsession with frogs.