The Shadow is a recurring villain in the Geronimo Stilton book and cartoon series. She is New Mouse City's most notorious thief, and Geronimo Stilton has come across her several times and foiled her plots, but she usually escapes. She is also Sally Ratmousen's cousin.
In the Shadow's first appearance, she targetted a museum. Dressing as a mummy, she scared off visitors so that everyone would be scared and she could rob the place in peace. Geronimo, joined by his nephew Benjamin and his love-interest's niece Bugsy Wugsy, arrived to inspect the museum to see if a mummy really was there. The Shadow dressed up as the cleaning lady as well as a mummy, but Benjamin and Pandora chased her and took off her disguise. She released a gas bomb, allowing her to escape.
Later on, she stole cheese from a cheese factory. She made a house out of the cheese, but Geronimo stowed away in a truck containing the cheese and ended up in the same house. He briefly spied on her and saw her talking on the phone with Sally. This would suggest that Sally too may be a criminal. She found Geronimo and knocked him unconscious before escaping.
Later on, she dressed up as the paleontologist Dr. Karina von Fossil and hitched a ride from a plane, along with Geronimo. She manipulated Geronimo into revealing to her that he was searching for a treasure. She then disappeared. She had her henchman, Bondi Bobirat, strand Geronimo and his family in the desert and stealing their truck, food, and water. Later on, when Geronimo was guarding a huge wooden box containing a Tarbosaurus fossil, the Shadow confronted him in her disguise before Bondi knocked him out. They tied him up and she revealed that she was the Shadow and planned on selling the Tarbosaurus to become rich. However, Geronimo's family, along with the Mongolian rodents, arrived and the Shadow and Bondi were arrested.