Rudgrin is the main antagonist of the novella The Ruby Princess Runs Away, or at least, its short film adaptation.
She is a supposed princess and the leader of the Darklings, and she plans on taking over the Jewel Kingdom as a replacement for the protagonist, Princess Roxanne of the Red Mountains. It is unknown why she plans on taking over the Jewel Kingdom.
Before Rudgrin (wearing a mask to look like Roxanne) gets crowned as the Ruby Princess, Roxanne returns, and then proves to her parents, sisters, and guests (with help from the royal servant Twitter) that she is the Princess (like her selflessness, and that she has the ruby necklace, and that it glows, not to mention she proves specifically to her Emerald sister Emily that she has red stains on her fingers from the two of them picking berries the day before). Then Twitter forces the mask off Rudgrin, and she runs away in anger, and then the King sends his captain after her.
Rudgrin is played by Molly Bauckham who did Roxanne's singing voice. Strangely, Rudgrin didn't sing.