Medusa is described as a greedy, obsessed, manipulative and duplicitous person. She is completely fixated on diamonds; when we first see her pawn shop, there is a prominent sign advertising half rates for people who bring in diamonds, and though she could have made a lot of money on the assorted pirate treasure in the cave, she brushes it off because the Devil's Eye, the world's biggest diamond, is the only thing that she is after. She is also very crazy, foolish, opprobrious, ill-tempered, vituperative and sometimes unstable, which makes her intimidating, treacherous, and dangerous at times.
However, she has a bit of a bumbling and unsophisticated nature, which makes a little hard for people to take her seriously as a really scary, ominous and threatening villain. She is very intelligent, rapturous and patient, but when she comes to the point of losing it, she is not someone that anyone wants to be around. She would stop at nothing to get what she wants; such as using the little girl named Penny, to go down into a dark cave just to retrieve the Devil's Eye.
Madame Medusa is aided in her misdeeds by two fearsome "pet" crocodiles named Brutus and Nero, that while being tame are also brutes who enjoy causing trouble - she also had aid from the blundering Mr. Snoops, who is extremely happy to go along with Madame Medusa because of his own greed: the two would often bicker however and their partnership is a very weak one.
Medusa is first mentioned in the movie by Penny's old brown cat named Rufus, who explains to two mice Bernard and Bianca (the heroes of the film) that a weird lady was always trying to give her a ride, but was always turned down. He then says that the lady and her partner run a sleazy pawnshop down the street.
Bernard and Bianca head over to the pawnshop and after being scared from a ringing phone, they hide to avoid being caught. Medusa then reveals herself, irritated at the phone ringing so late at night. She picks it up and is elated upon hearing the voice talking to her, which turns out to be her partner, Snoops, believing that he has found a diamond that she has been searching for. But instead, Snoops tells her to give him time to which Medusa revokes due to the fact that he had been looking for three months. Snoops then says something about "bottles" and Medusa concludes that he is talking about Penny sending message in bottles. She then launches a tirade on Snoops for his inability to "control a little girl" and says that she will be taking the next flight to Devil's Bayou before hanging up on him by way of slamming the phone.
She then grabs a suitcase, before heading to her room and comes back with a pile of clothes that she stuffs inside while huffing about Snoops's incompetence, then grabs a purple fur coat. As Bernard and Bianca try to get inside the suitcase, Medusa tries to shut it, but due to her careless packing, it doesn't. She furiously tries to shut it before jumping up and shutting it with her butt, which hurts her a bit. In her car, Medusa has trouble starting it due to Bianca being underneath the accelerator, but eventually gets going and drives in a Cruella-esque fashion (spinning around, jetting past a truck, and falling into a ditch). Her reckless driving causes Bernard and Bianca in the suitcase to fall out while she drives off.
At Devil's Bayou (presumably, a swamp in Louisiana), Penny tries another of many attempts to escape and Medusa realizes it when she calls her multiple times and she doesn't answer. She order Brutus and Nero to find the girl and bring her back. Meanwhile, Medusa plans to search for Penny in her swamp mobile. She tries to get it started, but cannot (mostly due to a lack of gas) and it ends up flying in the air before landing back into the water. Medusa looks back at the engine and grows furious at the fact that the vehicle won't start, shouting at the recently arriving Snoops as to what is wrong with swamp mobile. Snoops responds that he's not sure as the vehicle worked fine when he used it yesterday. After finally starting the vehicle, Medusa tells Snoops to shut up before ordering him to launch his fireworks to light up the bayou so she'll be able to find Penny and gets going in her swamp mobile.
Later, during her search, Medusa unknowingly gets Orville sucked into her swamp mobile, not noticing him as she continues her search. She uses one of the lights from her mobile to look for Penny and almost catches her hiding in the swamplands. Nero and Bruts manage to bring Penny back to Medusa's houseboat and Medusa learns about this after Snoops fires off some fireworks in the sky that read, "Got Girl". She returns back to the boathouse, flying over Bernard and Bianca before making a three-point landing, sliding into the boathouse, and making a skidding stop.
Returning back with a puff of black smoke behind her, she demands to know where Penny is and Snoops says he sent Penny to her room. Medusa asks Snoops in irritation why he let Penny escape and what alibi he has for her this time. Snoops blames Nero and Brutus before handing her one of her gallashes, which she throws back at him. She then compliments her pets for capturing Penny whom she calls an ungrateful little brat. Snoops complains to Medusa about using Penny for her caper and thinks she should leave it up to him, but she quickly cuts him off, telling him that he's not thinking. Snoops responds by saying he knows that Penny is the only one small enough to squeeze down the black hole.
Frustrated at Penny's inability to find the diamond, Medusa smacks her cane against Snoops's hands after he presents her with a handful of gems that Penny acquired during her trips to the cave. She then blames Snoops for not leaving Penny in said cave long enough. Snoops respond by saying that Penny won't take orders to which she responds by saying that he is just too soft on her. Snoops retaliates by saying that he had to take Penny out of the cave because the tide was coming and besides that, all Penny was doing was fussing about her teddy bear getting wet. Medusa then says that Snoops doesn't have a way with children; one has to gain a child's trust and make said child like them. Snoops asks how and Medusa angrily responds, saying that one forces a child to like them before calling him an idiot and almost hitting him with her cane. She concludes that when the next low tide comes in, she will put Penny in the cave herself and keep her there until she finds the diamond before walking off with her two alligators.
In her room, Medusa is looking over a map of the bayou when she hears the sound of organ music playing (Nero or Brutus playing it to get to Bernard and Bianca). Annoyed by this, she storms out and confronts Nero and Brutus, telling them to stop the racket before hitting on of them with her cane. This causes one of them to spit out Bernard whom they had just eaten and Medusa screams in terror upon seeing him, rushing to a chair, grabbing her dress, and telling Snoops to kill him.
As Snoops tries to kill Bernard with a broom, he accidentally hits the chair Medusa is standing on, breaking it and causing her to fall. She freaks out even more after seeing Bianca. Wanting the two mice dead, she grabs a shotgun and shoots at them, hitting Snoops and Brutus or Nero in the process. After wildly shooting around, Medusa's gun fails to shoot anymore bullets. She grows furious as she tries to shoot something out of it and asks Snoops what's wrong with the gun as she beats on it. As Snoops says that he doesn't know what's wrong, her gun shoots off some bullets which hit Bernard and Bianca as they are escaping.
Later in the night, Medusa calls Penny from her room, saying that she wants to talk to her. She is then seen putting on her nightgown and taking off her earrings, angrily saying to herself that high tide or not, Penny will find her the Devil's eye tomorrow or else.
The next morning, Medusa and Snoops bring Penny back to the cave, where Medusa steals her teddy bear and threatens to destroy it if Penny does not go down and find the diamond. After Penny finds the Devil's Eye (which was located inside a skull) and returns back up, Medusa take the diamond for herself, delighted that it is finally now in her possession. However, she refuses to return Penny's teddy bear and instead sows the diamond inside it. The greedy Medusa also betrays Snoops, despite his objections that he get half of the fortune for locating the diamond. Back in the houseboat, Medusa holds both Snoops and Penny in gunpoint, preparing to leave them behind. However, Medusa falls over a trip wire (planted by Bernard and Bianca) making her lose hold of the teddy bear.
As Snoops and Medusa engage into a fight to get the diamond, Penny grabs the teddy bear and (with the help from Bernard, Bianca, and several swamp folk animals) starts up Medusa's swamp mobile to escape. Medusa, Brutus, and Nero manage to catch up to them by grabbing hold of the rope, but Medusa starts whipping both alligators for no reason, much to their anger. It was not until Penny made a turn, causing the rope to snap and Medusa to crash on a smokestack as her houseboat has sunken thanks to the swamp animals firing up all of Snoops's firework supply.
As Snoops escapes on a board laughing at her misery while Penny and the animals head away from the scene, Medusa is now at a dead end by the wrath of both Brutus and Nero, who have finally had enough of her abusive, crotchety and vituperative behavior and attempt to devour her. Medusa is last seen holding her grip on the smokestack as she tries to avoid being attacked by Brutus and Nero, who intend and attempt to eat her alive afterwards. Her fate beyond this point is unknown, but it can be assumed that she either found a slim chance to escape her formerly loyal pets, or was eventually eaten by them.
- Due to an animation error, her skin turns purple occasionally throughout the film.
- Another error involved Medusa's left stocking being a lightish-brown color instead of a darker one; more specifically during the shot when she was petting Brutus and Nero just before Bernard and Bianca left the window.
- Despite Cruella De Vil was a recycled, yet deleted concept from the film, Medusa was created as a possible homage of her (which explains the design of her car).
- Although she is the main antagonist of the first film of the series, she is much less evil than her successor, Percival C. McLeach.