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 as well as being the artificial intelligence behind the Pillar of Autumn, and Cairo Station.

Appearance
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, a more realistic figure, making her more human, a more feminine figure, and a slightly different shade of purple.

Function
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 it's 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.

Creation
Cortana was actually created using a cloned brain of Dr. Catherine Halsey, the SPARTAN project creator. It is mentioned in Halo: Fall of Reach that she resembles a younger Dr. Catherine Halsey, with a similar attitude ‘only unchecked by military and social protocol’. Cortana's primary mission was to assist the SPARTANs in their mission to capture a Covenant Prophet and return him to Reach for interrogation.

It's very possible that she shares her creator's memories. During the opening sequence of Halo 3, Cortana is talking about how John is different, in that he has 'luck' on his side; one of the things that Dr. Halsey tested the children for is 'luck' (in Fall of Reach when Cortana picked the Master Chief, Dr. Halsey told her he was the luckiest Spartan). In the 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. In the hive that was once High Charity, you can hear her say the lines that Dr. Halsey had said to John when they first met on his homeworld. "May I speak with you, please? What's your name? It's very nice to meet you. So you like games? So do I." Even if the transcription of the Cortana at that moment were incorrect than the novel as it was in the Fall of Reach, it actually makes sense. Cortana, who was created from an imprint of Dr. Halsey's mind, would retain some of Dr. Halsey's memories. If that memory were to have been the most prominent during the creation of Cortana, it is entirely possible that Cortana would have that memory. This would also be supported by her statement "A collection of lies; that's all I am! Stolen thoughts and memories!" Later on in that same level, she says: "I'm just my mother's shadow..." Her mother meaning Dr. Halsey. At one point she also says that "It was the coin's fault," refering to the game of chance that Dr. Halsey played with the young John.

Personality
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. Her loyalty was brought into question after the events of Halo 2, where she accepts (but somewhat aggressively) to answer the Gravemind's questions. When she is reacquainted with the Master Chief in Halo 3, she admits to answering his questions and regrets it, and proves her loyalty by helping the Master Chief to contain the Flood. She has an unwavering loyalty to the Chief and is willing to do whatever she can to help him.

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 and maybe 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 the Halo 3 in the map High Ground inside the first one of the bunkers contains a screen that observes a corridor but 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.