On the green, I'm the boss!
~ Sally Ratmousen's motto.

Sally Ratmousen, known as Sally Rasmaussen in the television series, is one of the main villains of the Geronimo Stilton book and television series. She is the boss of the Daily Rat and is Geronimo's #1 rival. She is determined to prove the Daily Rat is better than the Rodents Gazette.

Sally Ratmousen is very cruel in her methods to prove she is better than Geronimo. In Paws Off, Cheddarface, she has another mouse look like Geronimo and ruin his life, turning his family and friends against him and having the imposter sell the Rodents Gazette to the Daily Rat. Eventually the Stilton family beat her at her own game by making Trap look like Sally. The imposter apologizes, since Sally tricked him into thinking it was just a prank.

Another of Sally's acts was in Valentine's Day Disaster, where she stops Geronimo's newspaper from reaching the public. When Geronimo aks if she can deliver it to the public, she refuses. It is also mentioned that Geronimo once accidently pushed her into a thorn bush, which might explain her hatred of Geronimo. At the end of the book, she agrees to send the newspaper to the public so that Geronimo will invite her to his party, hinting she has a softer side.

Sally has a cousin called The Shadow. The Shadow is a thief in New Mouse City. In The Mysterious Cheese Thief, she is on the phone with The Shadow, who has stolen from cheese factories, while Geronimo watches. This would imply that Sally may have a criminal record as well or is at least in on the plan.

She is also a golf champion. In The Giant Diamond Robbery, she competes with the Stilton family in golf. Her grandmother is Molly Ratmousen. However, The Shadow also comes and keeps Molly hostage and disquises herself as Molly to steal the Super Mouse Cup (this implies that she attacked her own cousin and accomplice, insuniating more that she no longer work together and that Shadow has lost has his scruples in attacking is own family)

. When she is discovered, even Sally is shocked. The Shadow manages to escape, and they continue the game and Geronimo wins, much to Sally's anger.