Canon Update: Or. So I don't want to play Portal 2
BACK GROUND
GLaDOS was taken from the end of Portal, where she was killed in a fire. After this an unknown amount of years have passed, but it is “hundreds of years later” possibly more then 1000. This entire time GLaDOS has been stuck in a loop of the last two minutes of her life. She is woken up by the same test subject that killed her. Once in control of the facility again, GLaDOS immediately starts the tests up again. However the subject has help, and with the aid of Wheatley manages to escape the testing chambers.
GLaDOS catches the subject in a trap but by that time the subject has already shut down the turrets and broken the flow of neurotoxin. GLaDOS is forcefully pulled out of control and replaced with Wheatley. It is at this time that she is put in a small potato battery. Wheatley however turns on the test subject and throws both her and Potato GLaDOS into an abandoned part of the testing facility. GLaDOS is then abducted by a bird.
The test subject eventually runs into GLaDOS again when she finds the birds nest. GLaDOS tells her that Wheatley was literally designed to be incredibly stupid and that he won’t know how to run the facility and that it’s going to explode if she isn’t put back in control soon. The only way for either of them to survive is if they work together. The test subject sticks Potato GLaDOS to the end of the Portal gun and they work their way through the old testing chambers. Along the way they hear pre-recorded messages from Cave Johnson, the founder of Aperture Science. GLaDOS starts to realize she knows him from somewhere, and when his assistant Caroline speaks up, GLaDOS echoes the woman’s words before stopping in terror, not knowing why she did that. She spends a lot of the time in the test chambers trying to figure out what’s wrong with her. She deduces that there is another voice….her own voice and it’s becoming something of a coincidence.
Inside the facility GLaDOS tries to fry Wheatley with a paradox but the AI is too stupid for that to work on him. They are currently working through some of the test chambers. The evidence of test withdrawal is starting to show in Wheatley. GLaDOS will be taken from this point.
Personality Change
When she wakes up at the beginning of the game GLaDOS seems even more bitter and angry after being forced to relive the last two minutes of her life for the past hundreds of years. She’s petty and insulting and anything she can say to insult the test subject she will. Going so far as to insult her weight, appearance, and status as an orphan. GLaDOS, holds grudges. While in the first game she’d pretend to be on the test subjects side, now she’s open with her hatred.
However GLaDOS also has a voice in her head now that she is in a potato. From what she can tell, it’s a woman named Caroline. However it is also her own voice. She suspects she was based on Caroline or that Caroline was forced into a computer. She’s still very confused about that. But GLaDOS has something of a conscience at the moment. She still hates the test subject but she’s starting to realize that the woman is the closest thing she has to a friend and that she doesn’t want to betray her when they manage to save the facility.
GLaDOS acts more human as well. She’s honestly terrified when she doesn’t know why she’s remembering these strange things and while her anger is still there, it’s focussed towards Wheatley and not on the test subject even though she spent the first part of the game wanting her dead.