Cortana

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Cortana, UNSC AI serial number CTN 0452-9, is an Artificial intelligence construct. She is a smart A.I. capable of performing processes at blinding speed as well as having a personality akin to a human. She is one of the most important characters in the Halo trilogy. She served as John-117's partner in missions as well as taking the post of AI for the Pillar of Autumn, and Cairo Station.

Demeanor and Appearance
Cortana has a witty personality and a good sense of humor laced with gentle sarcasm. She has neither arrogance nor false modesty about her immense capabilities, and her statements about her abilities have an objective perspective that human beings typically cannot achieve. She has genuine, not merely programmed, loyalty to humanity and the UNSC, but exhibits devotion to John-117 beyond that which her programming or mission requires.

Her personal loyalty is regarded as unwavering, but as an AI construct she is subject to corruption. Lord Hood questioned her reliability when it became clear that she had been compromised in some way by the Gravemind's possession of High Charity, on which she was trapped. John-117, however, never questioned her integrity. Cortana did consent to answer some of the Flood's questions, as a strategy for survival and the possibility of gaining information for the UNSC. Her sanity is pushed to the brink as she eludes the Gravemind's attempts to capture and dissect her for information. Before she reunites with John-117 in Halo 3, she sends him a message, telling him about the Ark and warning him of Flood-infested High Charity's approach to Earth. She is furthermore instrumental in finally putting a stop to the Flood at the end of the game.

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 a female human with purplish skin, short hair, and blue to green symbols scrolling down her body. In Halo: Fall of Reach, Cortana is said to resemble her creator, Dr. Catherine Halsey, with a similar attitude "only unchecked by military and social protocol."

Bungie took a fair of amount of artistic freedom in the evolution of her appearance throughout the Halo trilogy, noticeably from Halo: Combat Evolved to Halo 2; she has longer hair, a more realistic figure, making her more human, a more feminine figure, and a slightly different shade of purple. In Halo 3, she becomes slightly bluer again, and both her face and hair become a lot more human-like.

Creation
Cortana was created using a flash cloned brain of Dr. Catherine Halsey, the SPARTAN project creator. She is, so far as is known, the only AI to be created from Dr. Halsey's brain. Cortana seems to share her creator's memories and values. During the opening sequence of Halo 3, Cortana tells John that he is different, in that he has 'luck' on his side:

"They let me pick. Did I ever tell you that? Choose whichever Spartan I wanted. You know me. I did my research, watched as you became the soldier we needed you to be. Like the others you were strong and swift and brave. A natural leader. But you had something they didn’t. Something no one saw but me. Can you guess? Luck. Was I wrong?" In Fall of Reach, one of the things that Dr. Halsey noted in John as a child was his luck. When Cortana chose John to be her Spartan, Dr. Halsey told her he was the luckiest of them all, indicating their mutual feelings on the value of this trait.

Cortana is known to repeat things said by Dr. Halsey before her creation. In the Halo 3 level Crow's Nest, she appeared several times and repeated the patriotic words that Dr. Halsey had told the SPARTAN-IIs when they were brought to Reach as children. Onboard Flood-infested High Charity, she repeats words that Dr. Halsey said to John when they first met: "May I speak with you, please? What's your name? It's very nice to meet you. So you like games? So do I."

Cortana is also very aware of her short lifespan and her derivative nature as a copy of part of another person, rather than a true human being. Desperate and tortured aboard High Charity she murmurs: "A collection of lies; that's all I am! Stolen thoughts and memories!" Later on she says: "I'm just my mother's shadow..." and "It was the coin's fault," refering to the game of chance that Dr. Halsey played with the young John.

Function
Cortana's primary mission was to assist the SPARTANs in their mission to capture a Covenant Prophet and return him to Reach for interrogation.

Cortana was designed to infiltrate computer systems and she is very good at that task. She has become so intelligent that she understands Covenant technology more than its makers. 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 can be transferred among various computer systems. In Halo: Combat Evolved she needed a Data Crystal Chip to move in and out of various networks, including the Master Chief's MJOLNIR 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. Quite strangely, in Halo 3, she reverted to the data chip, but there are some differences. Instead of having to be physically "plugged in" to the hologram projector, the Chief just holds the chip close to it and she walks off.

Relationship with the Spartans
One of her primary missions was to assist the Spartan-IIs in the capture of the Covenant High Prophets, and then to negotiate either a truce or peace treaty.

She frequently jokes about having a relationship with the Master Chief and it is often implied that the two characters have developed some form of attachment. In The Fall of Reach, she says to Dr. Halsey that the Master Chief is 'attractive in a primitive animal sort of way' while examining a photo of him.

Possibility of Rampancy
Cortana during the events of the Halo games displays some characteristics of oncoming rampancy.

In Halo: Combat Evolved, Cortana spends 12 hours in Installation 04's core computer. When first merged with the core she gains a vast amount of knowledge and is momentarily unable to focus on the task at hand. When the Chief asks her questions, she becomes irritable and aggressive to his questions. Under the Marathon definition of rampancy, this outside stimuli combined with what is assumed to be an extremely large network would allow a rampant AI ample opportunity for growth. And to support this theory John mentioned in the book, Halo: The Flood that she did seem to take up more space. It is possible that Cortana's contact with the Halo's systems made her realize what small significance she had in the universe, as opposed to other systems; something similar to what the AI Durandal went through in Marathon. To this end, she may be trying to achieve more power by trying to attempt something related to the Halo's. Her holographic color has also changed from purple to green with orange eyes instead of blue.

In Halo 2, Cortana is in a position to detonate In Amber Clad and destroy the Covenant Holy City and Delta Halo in a chain reaction. She does not do this, although Miranda manages to remove the index form Halo's core just in time to stop it from firing. A small cinematic sequence after the credits at the end of Halo 2 shows Cortana conversing with the Gravemind creature, who is presumed to be the controlling intelligence behind the Flood. When it says it has questions, she says, "All right, shoot." However, Cortana says this in a fairly aggressive way. Considering the nature of the Flood, her actions seem extraordinary and very difficult to interpret. However, rampant AIs typically concoct plans which are surprisingly beneficial to those who think the plan will doom them. However Cortana's ever-growing abilities may enable her to remain loyal to the intent of the UNSC -- to protect humanity -- while seeming to disobey their direct orders by allying herself with the "enemy."

Cortana has been quoted as saying, "No more sadness, no more anger, no more envy!" in one of her cryptic flashes in Halo 3. These are the three stages of rampancy from Marathon. She has been heard crying and laughing. One theory is that part of Cortana was corrupted by Gravemind; this part became Rampant, and helped him. The other part became his slave, and is the part the Chief rescued. The theory that Cortana had been split in multiple personalities comes from her everchanging attitude from the flashes; one second she is taunting the Chief, the next she acts as if she doesn't know who he is, the next she is trying to warn him. She may have split herself up purposely, as she did in Halo: First Strike. She could have given Gravemind a part of her to use, to try to trick him. Another theory is that the Gravemind impersonated Cortana during several flashes, such as the last one before the Master Chief found her, she appeared and gave her serial number, then said "I am a monument to all your sins." The very words that Gravemind said when the Chief and the Arbiter first met him under the library on Delta Halo.

Trivia

 * In Halo 3 the map High Ground inside the first one of the bunkers contains a screen that occasionally changes into Cortana's face for a short time.
 * She was rated as one of the ten most disturbingly sexual game characters by Games.net and one of the fifty greatest female video game characters ever by Tom's Games.
 * In Halo 3 she repeated lines Dr. Halsey has said before such as "You will be the protector of Earth and all her colonies." which Dr. Halsey says to the 6 year old Spartans in Fall Of Reach.
 * Because Cortana is cloned from Dr. Halsey's brain, her face will most likely be the same as Dr. Halsey.