GLaDOS is the main antagonist of Portal M4R10 - If Mario was in... Portal. She is also the main antagonist in the Portal series, and claims that a Cake will be awarded to her Test Subjects at the end of the Test Chambers.
GLaDOS appears to be a robot attached to the ceiling of her chamber. She is a Genetic Lifeform and Disk Operating System, and as pointed in the name, is an AI. She has four Cores attached to her, these being the Morality Core, Curiosity Core, Intelligence Core and the Anger Core. They are all spherical objects, with their own special personalities installed within them.
GlaDOS is a psychopathic, manipulative and murderous robot that incapacitated all of the staff at Aperture Science. She then put them through her Test Chambers, which could lead to her potentially having an OCD with it.
Her Morality Core is silent, and seems to supress her powers, by preventing her to use the neurotoxin. However, it has a major flaw and allowed a Rocket Turret to try and kill Mario, but this plan backfired.
Her Curiosity Core is an inquisitive and questionable, and seems to give her a lust of curiosity and is one of her weaknesses, and GLaDOS tries to learn on what something does when it is attached to her.
Her Intelligence Core is obsessed with the Cake, and is potentially corrupted. It spurts out a random recipe for a Cake, which seems to contain ingredients that you do not want in your Cake, and it can probably kill you.
Her Anger Core is probably the most corrupt Core on GLaDOS, with the exception of the Toad Core. It probably causes GLaDOS to hate Mario and set up traps for him to go through, and probably adds the dangerous testing elements to her Test Chambers.
Portal M4R10 - If Mario was in...Portal
In Portal M4R10, instead of Chell being picked by Doug Rattmann, he picks Mario, probably due to his plain stupidity and his chance to destroy anything. When Mario makes up, he takes his name tag out, and GLaDOS says that he should not remove his name tag. He continues to try and find a way out, and GLaDOS questions if he is listening to her, which he replies by saying shut up to her. He then proceeds through the Test Chambers, until he reaches Test Chamber 16.
In the Test Chamber, GLaDOS ambushes Mario by placing a variety of Turrets, including defective ones. Mario then takes his clothes off, and it causes ERROR 69: TOO FUGLY to appear. They then explode. He proceeds to Test Chamber 17, and he is introduced to the Companion Cube. He falls in love with it, and imagines doing romantic things with it after he climbs up using it. GLaDOS then snatches the Companion Cube and prevents him from marrying it. She chuckles and chucks it down into an Emergency Intelligence Incinderator, and she states that she hates Mario and it cannot be for no reason, and he must deserve it.
Mario was not having it, and he would escape the Test Chamber by throwing a Defective Turret into the observation room, and he goes Behind the Scenes. GLaDOS states she can see him, but Mario changes the channel to Channel 4, which was showing Teletubbies. The camera explodes, and he progresses to her lair.
He confronts GLaDOS, and she congratulates him and asks him if it was worth it, despite his violent behaviour. She then announces that she has a surprise for him, and gives him the anticipated Cake, which he eats. She then deploys a Rocket Turret and tries to ambush Mario, but he dodges the rocket shot by the turret. He then proceeds to trick the Rocket Turret into looking at him naked, which causes it to explode. She then states she hates him, and he grabs onto the Curiosity Core and then he breaks and destroys it. She then turns on the neurotoxin, and Mario becomes intoxicated. GLaDOS manically laughs, and then he spots the Toad Core. Mario then flings himself onto GLaDOS once again, and attaches it. As a consequence, GLaDOS becomes 100% corrupt and explodes. She becomes detaches, and Mario celebrates this. The Announcer announces that the Core (which could be GlaDOS) was overheating, and a nuclear meltdown was imminent, and the entire chamber began to implode, and Mario, along with GLaDOS and her parts, are lifted up to the surface. Mario then wakes up back in the castle, and Peach says he baked a Cake for him. However, it looked exactly like the Cake GLaDOS presented, and he screams.