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 possessing a personality akin to that of a human. She is one of the most important characters in the Halo trilogy. She served as John-117's partner in various missions as well as taking the post of AI for the UNSC Pillar of Autumn, and the UNSC Orbital Defense Platform Cairo Station.

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.

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

Halo: Combat Evolved
Controlling the Pillar of Autumn 's defenses, Cortana destroys three Covenant targets over Installation 04 before the ship's weapons are disabled. Under the Cole Protocol, Captain Keyes, the Autumn's commanding officer, prepares to abandon ship. Since the protocol mandates the evacuation of any AI constructs, the Master Chief is charged with safeguarding Cortana from the Covenant. When the Master Chief arrives on Halo, Cortana helms the communications channels and helps to direct Foehammer, a dropship pilot, to human survivors scattered across the ring, and assists the Master Chief in the rescue of Captain Keyes from the Covenant ship Truth and Reconciliation.

Inserted into Halo's Control Room, Cortana looks for a way to activate Halo to use as a weapon against the Covenant, but becomes visibly agitated and sends the Master Chief to find Captain Keyes. Cortana stays in Halo's computer core as the Master Chief encounters the parasitic Flood and is conscripted by the Forerunner construct 343 Guilty Spark to activate Halo's defenses.

The Master Chief and Guilty Spark return to the Control Room, intent on using the Index, the key to Halo, to eliminate the Flood, but Cortana reveals the truth that she has learned: Halo does not kill Flood, but their food. If Halo were activated, all sentient life in the galaxy would be destroyed. She takes the Index and thus becomes a target for 343 Guilty Spark. Having captured the Index, Cortana and the Master Chief plan to destroy Halo. They succeed after Cortana helps the Chief to detonate the Autumn's fusion reactors, causing an explosion powerful enough to destabilize the ringworld. The two escape in a fighter starship and witness the ring's destruction.

Halo: First Strike
In the third novel, Cortana and the Master Chief, seemingly the sole survivors of the events of Halo, discover a small number of other United Nations Space Command (UNSC) personnel have in fact escaped the ring. Cortana helps to take control of a Covenant cruiser, Ascendant Justice, and later returns to Earth with the remaining survivors after destroying the Covenant space station Unyielding Hierophant.

In this novel, Cortana gains the ability to create imperfect clones of her program. A clone that the Master Chief and his Spartan Blue Team bring to Unyielding Hierophant eventually re-clones itself hundreds of times to aid the Spartans in completing their mission.

Halo 2
Cortana appears next in Halo 2, on the Earth defense platform Cairo, at an awards ceremony for the heroes of Halo: Combat Evolved. When a Covenant fleet arrives, Cortana takes control of the Cairo's Magnetic Accelerator Cannon to repel the invaders, and successfully deactivates a bomb that would have destroyed the station. Later, upon discovering Delta Halo, Cortana gives Commander Miranda Keyes access to all information on the original Halo, and provides intelligence to the Chief and UNSC Marines on the surface of the ring.

When sent by the Flood leader, Gravemind, to the Covenant city of High Charity, Cortana stays there as the Master Chief follows the Prophet of Truth. She promises to detonate the crashed In Amber Clad's reactors to destroy the city and Halo if the ring is activated. The firing of Halo is averted by Keyes, the Arbiter, and Sergeant Major Avery Johnson, but Cortana is left with Gravemind, who has overrun High Charity.

Halo 3
Throughout the majority of Halo 3, Cortana remains in the clutches of the Gravemind. Throughout the campaign, she sends a series of increasingly disturbing messages to the Master Chief, during which she quotes several lines of Halo literature and alludes to other events.

In these broken transmissions, 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 manages to send a message to the Master Chief on Earth, through a Flood-infected ship. In her message, she states that Gravemind is unaware of the portal opened by a Forerunner artifact on Earth. Cortana continues to appear to the Chief, who later recovers her from Flood-controlled High Charity. Surprised that the Chief has against all odds rescued her (as he promised in Halo 2), Cortana produces the Index from Installation 04, which she has kept as a souvenir. With it, Cortana is able to activate a new ringworld being constructed.

While the Flood are destroyed as planned, a slipspace portal collapses as the Master Chief, the Arbiter, and Cortana attempt to escape through it, thus stranding the Chief and Cortana. Cortana activates a distress beacon, but knows that it could be years before they are found. As the Master Chief prepares to go into cryonic sleep to await rescue, Cortana confides to him that she will miss him. He replies to wake him when she needs him.

Personality
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 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.

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.

Rampancy
During the events of the Halo trilogy, Cortana displays some characteristics of oncoming rampancy. In the gap between the release of Halo 2 and 3, there was much speculation to this effect.

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.

Appearence
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.

Abilities
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.

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 similar or the same as Dr. Halsey.