GLaDOS

"Why do I hate you so much? You ever wonder that? I'm brilliant. I’m not bragging. It's an objective fact. I'm the most massive collection of wisdom and raw computational power that’s ever existed. And I hate you. It can't be for no reason. You must deserve it."

- GLaDOS, to her sworn enemy

GLaDOS (Genetic Lifeform and Disk Operating System) is an artificial intelligence from the Portal series. She is the main antagonist of Portal, forcing our hero, Chell, to go through a series of pointless tests that ended with GLaDOS trying to kill her. In Portal 2, however, she was much nicer, instead helping destroy Wheatley during the second half of the game. At the end of Portal and Portal 2, she sang the songs Still Alive and Want You Gone (respectively), which was loved by fans of the series.

GLaDOS was based off of the personality of Caroline, the secretary and close assistant of Aperture Science CEO Cave Johnson. She becomes much nicer under the influence of her original personality, but ultimately deletes it at the end of the game.

Portal
GLaDOS is the main antagonist in Portal. She guides the player throughout a series of tests, appearing to be a series of pre-recorded messages. However, some of these messages have an odd note to it (messages shorting out at important times, not reacting when Chell is clearly in mortal danger), hinting that something strange and possibly sinister is going on.GLaDOS motivates her test subject with the promise of cake and grief counseling once the tests are over. However, any rewards she promised are, in fact, lies. Instead of the aforementioned rewards at the end of the testing tracks, she instead sends the subject on a moving platform headed towards an incinerator.

The protagonist of the game, Chell, manages to outsmart GLaDOS and escape, prompting her to be more emotionally expressive, showing alarm and claiming that she is "pleased that [Chell] made it through the final test where we pretended we were going to murder you." As Chell's escape progresses, GLaDOS finally shows her true colors and declares: "Turn back or I will kill you. I'm going kill you, and all the cake is gone."

When Chell finally shows up to confront GLaDOS in her lair, the latter tricks her into incinerating her Morality Core, freeing her from its restraints and allowing her to flood the Enrichment Center with a deadly neurotoxin. The Morality Core also had a few ancillary responsibilities, as GLaDOS was incapable of shutting off the Rocket Turret that had been activated upon the core's destruction. This is a key component for defeating her in battle, as Chell must redirect the rockets to knock Personality Cores off of GLaDOS and incinerate them. Once that has been acheived, they are both blown up to Aperture's parking lot, but before Chell can make her escape, she is dragged back into the facility by a Party Escort Robot.

After a quick tour of Aperture's interior, the player will see a room, with a Weighted Companion Cube and a cake. Then, multiple Personality Cores light up, and then a robotic arm drops down and extinguishes the candle on the cake, which was not a lie after all.

Portal 2
Since GLaDOS was destroyed during the events of the first game, Aperture Science has gone to ruin without her influence. When Chell and a rogue personality core reactivate her in a failed attempt to escape the facility, GLaDOS captures Chell again, but this time she has an agenda: revenge. When Chell eventually escapes the test chambers and rejoins Wheatley, the duo soon track her down in her lair and hijack her mainframe, putting Wheatley in control of the facility.

When Wheatley goes mad with power, GLaDOS is forced to work with her former enemy to restore control to the facility. Although she is initially reluctant towards this partnership, she quickly decides that they must cooperate if they want to survive.

When GLaDOS regains control of the facility, she lets Chell go as a reward, and GLaDOS is presumably left to do her tests in peace for as long as she likes.

Personality
GLaDOS can be described as openly sadistic, often making jokes about or simply having no reaction at all to situations that could harm test subjects to a major degree. She enjoys using reverse psychology and cruel jokes to discourage Chell throughout Portal, although her attitude towards Chell softens somewhat during the events of Portal 2. Whether the experiments she conducts on individuals within the facility have any use or are just done for the sake of it are unknown, but she has a sincere passion for science, which she regards as her sole purpose in her A.I "life."