Gus is a mouse and one of Cinderella's loyal companion.
Gus, or Gus Gus, is Jaq's best friend and rather dimwitted compared to his buddy. Other than his friends and family, his number one priority is making sure his belly is always treated right. Despite his small size, he has a tough mind and a running gag shows him putting up his fists if Cinderella is ever threatened only to be stopped by Jaq. He is the main target of Lucifer, who prefers Gus out of all the mice because of his plump size and slow composture.
In the first film, Cinderella Gus appears in the first scene of the film, trapped shivering in a cage where Jaq finds him and brings Cinderella to rescue him. Cinderella names him "Octavius," and calls him "Gus" for short. This is an example of an inside joke among the writers and (some) viewers of the movie. Gus is actually short for Augustus. In ancient Rome, Octavius was renamed Augustus when he became Caesar. Hence Octavius - Augustus - Gus. Jaq explains the situation to Gus, that Cinderella wants to help him, and Gus catches on and joins the mouse troop. Gus has a large belly that pokes out of his yellow shirt and seems to be a bit slow on the pickup, but brave in a pinch, and speaks even more broken English than Jaq, who calls him "Gus-Gus". Gus and Jaq are responsible for stealing the key to Cinderella's room from the wicked stepmother, Lady Tremaine, and freeing Cinderella to try on the glass slipper, which of course fits. While trying to steal the key, Jaq is stuck in Lady Tremaine's pocket and gets knocked around as she pats the key. Gus gets trapped in a tea cup and Lady Tremaine offers some tea to the Duke, with Gus inside. She begins to pour tea into the cup and it drops towards Gus's large belly. Gus sucks in his belly to survive, trembling and sweating, but in a moment the Duke says he wants none, the tea lifts, and Gus is safe (though he gets his butt burnt by the tea later when they get the key). They are last seen in the film in tiny palace uniforms, eating and waving at the wedding of Cinderella and Prince Charming.
In Cinderella II: Dreams Come True, Gus was also present, living in the castle along with all the other mice. It was Gus' idea to make the new book of stories to replace the old one. In Cinderella's Story, Jaq and Gus helped Cinderella to prepare for the ball and gave her support when she doubted her abilities.
In Cinderella III: A Twist in Time, Gus and Jaq make their opening appearance helping the fairy godmother prepare for the prince and Cinderella's anniversary party. After Lady Tremaine reversed time and made the slipper fit Anastasia's foot, they helped Cinderella search for the prince, (wreaking havoc in the palace kitchen in the process). They discover Lady Tremaine's use of dark magic by witnessing her cast a spell on the prince to forget Cinderella (and think that he danced with Anastasia). In a plan to steal the wand back, they succeed in taking the wand from Lady Tremaine, but she has Cinderella arrested with a banishment sentence before she can lift the spell on the prince. Gus and Jaq manage to convince the prince that Cinderella is the right girl through a catchy showtune, "At the Ball". They later help Cinderella put on her wedding gown, but the three are magically sent away by Lady Tremaine, who assigns Lucifer the cat to drive them in a pumpkin carriage over a cliff. After they narrowly escape their deaths, Cinderella, Jaq, and Gus return to the palace where the mice attack the entrance guards to let Cinderella through to the wedding. The mice are last seen in Cinderella III during the closing credits in a picture with a very decorated king, one with them stuffed with cheese, Gus in a rather sophisticated uniform, and the final picture in a photo booth style series with Cinderella, the prince, and the two mice.