4. Cortana from Halo

The perfect foil to the quiet powerhouse that is Master Chief, Cortana is unbelievably  intelligent, as heroic as any soldier, and to many gamers, a strangely attractive character for something made of light and programming. One of the only characters to appear in almost every major piece of Halo fiction, Cortana was originally created off of a template of Catherine Halsey, the creator of the Spartan II program. Later, Cortana’s “smart AI” capability allowed her to expand beyond her original programming. Even as Master Chief pulls off the heavy lifting that is required to save the universe from annihilation, Cortana works behind the scenes as an equally important component of the partnership. Her wry wit and keen insight make her an ideal companion throughout the Halo games, and secure her a spot as one of the most interesting female characters in games – even if she is just a program.

3. KOS-MOS from Xenosaga

Amid the strange philosophical and religious overtones of the Xenosaga series, one character rises above as a powerhouse that can’t be forgotten. The android shell of a teenage girl holds within it the power to save the universe from destruction, and players are treated to hints of that power throughout the course of Xenosaga’s lengthy story. Even as she summons gargantuan guns and devastating swords to bring down her opponents, an air of mystery always surrounds the character. As the series continues, one of the central reasons to continue onward is to answer the questions that hang around KOS-MOS. By the end, this blue-haired heroine plays a central role in the mind-twisting conclusion, and fans’ questions are finally answered.

2. HK-47 from Star Wars: Knight of the Old Republic

Say the word “meatbag” to any gamer and ask what they think of, and many of them will recall fond memories of a certain assassin droid from KoTOR. HK-47 offers a strange form of comic relief throughout the excellent BioWare RPG, declaring his hatred for organic life, and his desire to snuff it out. The only check on his sociopathic tendencies is the loyalty he holds to his creator, Revan. As players journey the galaxy, HK-47 can always be counted on to offer a violent and messy solution to the problem. Play on the light side, and he is a perfect counter to your heroic actions. Play on the dark side, and he probably has some excellent suggestions to keep things rolling along.

1. GLaDOS from Portal

As sentient, menacing computers go, it’s hard to beat GLaDOS. Take Hal-9000 of 2001 fame, and add a sarcastic mean streak and a punishing sense of humor, and you get the hilarious and dangerous persona of Portal’s central antagonist. Constantly goading the player into life-threatening trials, GLaDOS’ quips establish the personality that help transform Portal from a simple puzzle game into a timeless classic. As the game concludes, she voices the now familiar strains of “Still Alive”, reassuring players that she’ll return for another installment, and cementing her position as the greatest AI character in years.

Honorable Mentions:

Jehuty from Zone of the Enders
Chibi Robo from Chibi Robo
Legion from Mass Effect
Jack from Gears of War
Clank from Ratchet & Clank
Glitch from Metal Arms: Glitch in the System
Astro Boy from Astro Boy: Omega Factor
FATE from Chrono Cross

For more coverage of our top characters of the decade, check out our cover story hub.