Cortana

Summary
Cortana is a character in Halo: Combat Evolved and Halo 2. Cortana is a "smart" AI, meaning she has the ability to learn and adapt, but also making her capable of going rampant. She is the artificial intelligence behind the Pillar of Autumn, although she does not have absolute control; Captain Keyes retains some exclusive powers, which he can execute through his neural implants. This is likely used as a failsafe should rampancy ever occur.

Cortana has no physical body, but she can communicate through comm systems and project a holographic image of herself from appropriate projectors, such as Holotanks. Her chosen avatar is that of a female human with purplish skin, short hair, and blue to green symbols scrolling down her body. Her appearance changes noticeably from Halo: Combat Evolved to Halo 2; she has longer hair and a slightly different shade of purple.

Cortana can be transferred among various computer systems. In Halo: Combat Evolved she needed a computer chip to move in and out of various networks, including the Master Chief's armor. In Halo 2, she merely needs to be touching another system to access it; however, this is most likely a product of the Master Chief's upgraded armor.

Cortana was designed to infiltrate computer systems, and she is very good at that task. So far, she has not encountered a security program she was not able to defeat. In addition, she is proficient with UNSC military hardware, and can run military ships or stations by herself. Indeed, the UNSC's primary use of AIs is to oversee targeting of point-defense weaponry used in space battles; for Cortana, this is merely an additional capability.

Cortana was actually created using a cloned brain of Dr. Catherine Halsey, the SPARTAN project creator. Cortana's primary mission was to assist the SPARTANs in their mission to capture a Covenant Prophet and return to Reach for interrogation.

Cortana has a smart, witty personality and a good sense of humor. She has hardwired loyalty to humanity and the UNSC, but seems to be genuinely loyal in any case.

Cortana is voiced by actress Jen Taylor in both Halo games.

Quotes

 * "Keep your head down! There's two of us in here, remember?"
 * "If I still had fingers, they'd be crossed..." (during Bumblebee descent to Halo in Halo 1)
 * "Unfortunately for us both, I like crazy!" (during final cutscene of Cairo Station in Halo 2)
 * "You can just float and sputter!" (to 343 Guilty Spark)
 * "Don't make a girl a promise...if you know you can't keep it." (to Master Chief as he starts to say he'll return)