Cortana

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"A dozen CCS-class Cruisers against a single Halcyon Cruiser, I'm content in having 3... make that 4 kills."

- Cortana to Master Chief in the the beginning of Halo: Combat Evolved

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 speeds. She can calculate up to one trillion computations, 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 combat missions as well as serving as the AI for the United Nations Space Command Pillar of Autumn and the Orbital Defense Platform Cairo Station.

Creation
She was created using a flash cloned brain of Dr. Catherine Halsey, the SPARTAN project creator. She is, so far as is known, one of only two AI to be created from Dr. Halsey's brain, the other being her predecessor, Kalmiya. Cortana seems to share her creator's memories, thoughts and even values. Being a smart A.I., however, her operational life span is approximately 7 years. She is not only a facsimile and dubious dummy of Dr. Halsey's brain but also her physical form, at perhaps a younger age.

Fateful Meeting
Cortana's primary mission was to assist and help the SPARTAN IIs in their mission to capture a Covenant Prophet and return him to Reach for interrogation. She had a chance to pick which Spartan she would partner up with and assist. The Spartan she picked was John-117 because of his "luck". It should also be noted that she could of have picked John-117 because Dr. Catherine Halsey had always favored John.

Discovery
When the operation was canceled due to the surprise assault on Reach, she and her new partner, John-117, were left to flee the burning planet to a randomized set of coordinates per the Cole Protocol. John-117 thought the coordinates were strangely familiar; Cortana scanned them and found out that the coordinates matched the rock found on Sigma Octanus IV and the Covenant ship John-117 had boarded in 2525. After emerging out of slipspace, they discover a ring shaped world which was promptly named "Halo". However, Covenant ships followed, and began an assault. Controlling the Pillar of Autumn's defenses, Cortana destroys four Covenant targets over Installation 04 before the ship's weapons are disabled. Under the Cole Protocol, Captain Jacob Keyes, the Pillar of Autumn's commanding officer, prepares to abandon ship. Since the protocol forbids the capture or destruction of any A.I. construct, the Master Chief is charged with protecting 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 the Halo's defenses.

The Master Chief and 343 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 detonate the Autumn's fusion reactors, causing an explosion powerful enough to destabilize the ringworld. The two escape in a Longsword and witness the ring's destruction.

Dust and Echoes
After the destruction of Halo, Cortana and the Master Chief, seemingly the sole survivors of the tragic events on the ringworld, discover a small number of other UNSC personnel have in fact escaped the ring in a Pelican dropship. Cortana helps to take control of a damaged Covenant cruiser, Ascendant Justice, returns to Reach and fights the Second Battle of Reach, rescuing survivors of the glassing of the planet. Ascendant Justice was effectively fused with the UNSC Gettysburg and She later returns to Earth with the remaining survivors after destroying the Covenant space station Unyielding Hierophant that threatened Earth. (This is also the first time she encounters the Brute species.) In this instance, 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 completion their mission.

First Battle of Earth
Cortana returns to Earth, where she is on the Earth defense platform Cairo, at an awards ceremony for the survivors of the first Halo ring. When a Covenant Fleet arrives, Cortana takes control of the Cairo's Magnetic Accelerator Cannon (or MAC) to repel the invaders, and successfully deactivates a bomb that would have destroyed the station. The same bomb is used by Spartan-117 to destroy a Covenant cruiser.

Delta Halo
Later, after escaping on-board the In Amber Clad in a slipspace event, they discover another ringworld, Delta Halo. Cortana gives Commander Miranda Keyes access to all information on the original Halo ring, and provides intelligence to the Chief and UNSC Marines on the surface of the ring.

After the Prophet of Regret's assassination by the Master Chief, she is captured along with John by the Flood leader, the Gravemind. However, the Gravemind wishes not to harm them, but to stop the ring from being fired by the Prophet of Truth. She and the Master Chief are sent to the Covenant holy city of High Charity, Cortana infiltrates the city's computer systems and stays there while 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. However, she is left behind as a fail safe because she doesn't want to chance a remote detonation. The firing of Halo is averted by Keyes, the Arbiter, and Sergeant Major Avery Johnson, but Cortana is left in the clutches of the ally turned enemy, Gravemind, who has overrun High Charity.

Captured
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. The newly constructed ringworld, Installation 04 (II), is fired prematurely and destroys itself, and The Ark.

While the Flood are destroyed as planned, a slipspace portal collapses as the Master Chief, the Arbiter and Cortana attempt to escape through it, cutting the UNSC Forward Unto Dawn in Half. While the Arbiter (who was in the bridge) escapes to Earth through the portal, the Chief and Cortana (who were in the hanger) are left drifting in space. Cortana activates a distress beacon, but knows that it could be years before they are found. As the Master Chief prepares to go into a cryogenic 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, but she does get bored often and easily. 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," referring 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 Halo: 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.

Possible Rampancy
During the events of the Halo trilogy, Cortana displays some characteristics of oncoming AI rampancy. In the gap between the release of Halo 2 and Halo 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 Halos. 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 Commander Miranda Keyes manages to remove the Index from 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. 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 (moments) 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 ever changing 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." These are the very words that Gravemind said when the Chief and the Arbiter first met him under Delta Halo's Library.

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 appears to be an almost-naked, teenage, female human with purplish/bluish skin, short hair and blue to green symbols scrolling down her body. In fact, she does wear a skin tight apparently full-body suit which appears to change between the games. In Halo: Fall of Reach, Cortana is said to resemble her creator, Dr. Catherine Halsey as a teenager, 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 much 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 more realistic.

Abilities
Cortana was designed to infiltrate computer systems and she is excellent 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. She helped out in several major battles including Operation First Strike.

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, this was probably because the Master Chief's armor was badly damaged during the fall to Earth and reverted to a back up system, this is also why the shield is different. However, 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 and she "touches" the chip to return, holographic representations of the data transfer.

Also Cortana appears to have an ability exclusive to herself and 343 Guilty Spark (she may have gained it while in Halo's core.). This ability being able to hold actual solid objects (such as the Index) even though she is a merely a holographic form that should logistically not be able to hold solid objects. Although this may be possible if Cortana's image is displayed with light similar to Light Bridges.

Trivia

 * Cortana is voiced by Jen Taylor, the same actor that voiced Princess Peach in the Mario games.
 * In Halo 3, look for one of the UNSC mobile computers. Watch the lower-left monitor. After a few seconds, Cortana's face will appear for an instant and then disappear. Two of the easiest ways to see this is to either look at the aforementioned screen in the beginning of Crow's Nest, or at one of the computers in the multiplayer map High Ground.
 * There was an alternate story in Halo: Combat Evolved in which when Master Chief rediscovered Cortana inside the control room, she had became mad with power and wanted to take over Halo and the universe. Jason Jones revealed this on the Halo 3 Legendary Edition bonus disk.
 * Originally, Cortana was supposed to have a British accent. This was revealed by Joseph Staten in the Halo 3 Legendary Edition bonus disc.
 * 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.
 * Cortana's hair seems to get longer in each Halo game.
 * 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 Halo: The Fall Of Reach. Cortana also recites a few lines from the Cortana Letters in Halo 3 in some of her messages and in the announcement trailer.
 * Because Cortana is cloned from Dr. Halsey's brain, her face is most likely similar to or the same as Dr. Halsey's.
 * Cortana's color changes in each game.
 * In Halo 2, through modding, it can be found she has animations for some guns, although they are very simplistic variations of her standard pose.
 * Cortana has been through 7 Notable Battles
 * In Halo 2 and Halo 3, Cortana and Miranda Keyes have a similar hairstyle.
 * When Cortana speaks to you in Halo 3's level Crows Nest, notice how she says stuff that Dr. Halsey said in the book Halo The Fall of Reach.