GLaDOS

GLaDOS (Genetic Lifeform and Disk Operating System) is the main antagonist of Portal and is the main antagonist of Portal 2 for the first half of the game. She is an AI robot that presides over the Aperture Science testing center, and is one of the more well-known characters of the franchise, beloved for her dry sense of humor and sadistic personality, both of which shine especially in Portal 2. However, the way she gets humanized and sexywomanned is considerably eclipsed by how Wheatley was sexymanned, even now when the latter's sexyman treatment has died down and her own sexywoman status has risen. As such, she's only Euclid.

Basic Information
She starts out in the first game as seemingly benign, simply asking the player/Chell to go through tests of the portal gun technology. However, after you finish what's supposedly the last test, it's revealed she's actually malevolent and out to kill you, which is rather obvious given that she drops you onto a conveyer belt leading into a burning fire. After being defeated at the end of Portal, she returns in the second game, when you and Wheatley accidentally reactivate her, and she immediately begins putting you through tests again. But, when Wheatley is put into control, turns on Chell, and turns GLaDOS into a potato battery, the two are forced to work together to take him down. As they explore the depths of Aperture, it's revealed through audio recordings that GLaDOS' consciousness is based on Caroline, the former assistant to the CEO of Aperture Science, Cave Johnson.

Distinctive Voice
At least half of GLaDOS' appeal comes from her excellent voice work - there's just something about Ellen McLain's dry tone with the robotic effects that make it so memorable.

Quotable Catchphrase(s
Oh, she has plenty. For a start...

"Okay, look, we've both said a lot of things that you're going to regret. But I think we can put our differences behind us. For science. You monster."

Dominating
As the omnipresent AI running Aperture, she makes her presence known, especially in Portal 2, where she spends a lot of time insulting the player.

Nonhuman
Robot.

Power
The fact that she has complete control over the very floor you walk on and has a supply of deadly neurotoxin readily available at all times make her very intimidating, to say the least.

Knifemurder
Had absolutely zero issue murdering dozens of scientists when she took over Aperture, and after the events of Portal, makes it very clear she wants to kill you as revenge in Portal 2.

Well-Dressed
When she's humanized, her outfit can vary, but it tends to be something sciency like a labcoat. Speaking of which...

Humanization
Tends to get humanized as a rather attractive white woman - though, unlike Wheatley, her humanizations tend to keep part of her original robotic look (I.E. black and yellow eyes) to emphasize her cold personality.

Villain
Murderous AI - albeit a very funny one - definitely counts.

Intelligence
Incredibly so, thanks to being a supercomputer with access to tons of data at all times. Even when she's stuck as a potato battery she's pretty clever.

Angst
Her backstory as Caroline lends itself nicely to tons of fanfics and fanworks.

Theme Song
Still Alive, Want You Gone, and if you want to count Lego Dimensions, You Wouldn't Know.