Cortana

"This galaxy is vast; its wonders and beauty are almost unfathomable. But the galaxy also hides dark secrets, some of which have lain dormant since the beginning of time itself. There is a danger in secrets, both in seeking and in knowing. Some things are meant to be hidden from view. Some mysteries defy understanding, and sometimes even the things we think we know are untrue. Some secrets should remain untouched."

- Cortana

Cortana (UNSC Artificial Intelligence serial number: CTN 0452-9) is a UNSC "smart" AI. She was one of the most important figures in the Human-Covenant War, and 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, the Orbital Defense Platform Cairo Station and the UNSC frigate In Amber Clad. In addition, she held vital data pertaining to the Halos, including the Activation Index from Installation 04.

Creation
Cortana was created specifically for Operation: RED FLAG, a SPARTAN-II mission to capture a Covenant ship, and kidnap a high-ranking member of the Covenant hierarchy. Cortana was to serve as the codebreaker, hacker and intelligence lead in the mission, and an exceptionally intelligent human brain was needed to create the AI construct. It was decided that Dr. Catherine Elizabeth Halsey would the best possible brain donor. Under normal circumstances, the brain donor for an AI construct is dead, but Halsey's brain was flash cloned for the procedure. Twenty flash cloned brains were created, but only one survived; the survivor was used to create Cortana. The neural pathways of the cloned brain were scanned and copied through a process called Cognitive Impression Modeling. Her first words were, "Quando il gioco è finito, il re e il pedone vanno nella stessa scatola.", which translates to "When the game is over, the king and the pawn go in the same box." The same phrase was spoken by Halsey's mother after their first game of chess.

She is one of two AI that were created from Dr. Halsey's brain, the other being her predecessor, Kalmiya, another smart AI. She chose the name "Cortana" herself, and although Halsey did not tell Cortana that she had planted a viral termination code in her kernel, Halsey speculated that Cortana had already isolated and bypassed the code without her knowing. Cortana spend the first three years of her life residing in the computers of CASTLE Base, often accompanying Dr. Halsey in her work. Halsey enjoyed Cortana's presence, because she considered her an intellectual adversary, and also saw herself in Cortana; indeed Cortana is not only a facsimile of Dr. Halsey's mind, but also a replica of her physical form, albeit at a younger age. In Fall 2552, in preparation for the mission into Covenant-controlled space, Cortana was asked by Dr. Halsey to pick a "carrier". Cortana chose John-117 for their neural compatibility, although sharing Halsey's thoughts, considered him "attractive in a primitive animal sort of way".

Fragmentation and fateful meeting
"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."

- Cortana

On August 3rd, 2552, Dr. Halsey separated Cortana into two "fragments". One of these fragments, which contained all of her higher functions and personality, was stationed on board the UNSC Pillar of Autumn, while the other was tasked with mining data from the Forerunner artifact beneath SWORD Base. As the translation began to progress, Cortana's decryption of the Babd Catha artifact began to reveal what Halsey's fellow xenoarchaeologist, Laszlo Sorvad, had reported of the excavation, a "latchkey discovery", something that could change the tide of the Human-Covenant War. Deciding to ensure the data's delivery to Spartans, Dr. Halsey chose to recall NOBLE Team, who were not being "kept in the dark" about the Fall of Reach, and task them with delivering Cortana's fragment carrying the data to the Pillar of Autumn. Before their arrival, by August 14, Cortana and Halsey discussed which of NOBLE's Spartans she would trust to escort her, eventually choosing SPARTAN-B312, "Noble Six", noting the Spartan's "hyper-lethality", which she compared to that of John-117. However, contacting NOBLE Team proved difficult, as the team had been separated following the aftermath of Operation: UPPER CUT and would not be recovered by the UNSC until August 26th.

Meanwhile, on August 29th, 2552, the alternate fragment of Cortana was inserted into John-117's MJOLNIR Mark V armor for the first time, and the two then took part in a test to determine the abilities of their bond as an AI-human team. The test was devised by Colonel James Ackerson, a bitter rival of Dr. Halsey's who held secret contempt for the SPARTAN-II Program, and by extension, John-117. Ackerson incorporated armed Orbital Drop Shock Troopers, Lotus anti-tank mines, automated turrets in a razor wire field, and even a SkyHawk jumpjet for an air strike into the test in an attempt to have John killed. Despite the obvious fact that the test was sabotaged, John-117 was determined to finish without breaking the rules. With Cortana's assistance providing battlefield intel and by speeding up his reaction time to near-instantaneous limits, John managed to traverse the course relatively unharmed.

After the test, Cortana was installed into the as the primary AI, with another AI, Wellsley, as backup. While aboard, she exacted revenge upon Colonel Ackerson by hacking into his system and blackmailing him, hoping that this would send him back to the front lines. She also hacked into the SPARTAN-II highly classified personal files, and read over John's specifically. Cortana was horrified to discover the things Halsey had done to John and the other Spartans, and secretly vowed to protect him to the best of her ability. On August 30th, as the Autumn was making final preparations for the mission into Covenant space, Captain Jacob Keyes was informed that a massive Covenant fleet had dropped out of slipspace and were moving towards Reach. The Autumn was forced to abandon its original mission and to assist in defending Reach.

Fall of Reach
"So, you've made your choice? Yes, well... great minds do think alike."

- Dr. Catherine Halsey, agreeing on Cortana's choice of Noble Six as her carrier.

On August 29th, 2552, NOBLE Team arrived at Sword Base under pretense orders of a "torch-and-burn" operation to destroy SWORD Base, not finding out until midway through the battle that their real mission was to deliver the Package containing Cortana's fragment and the artifact's data to the Pillar of Autumn, which had docked at the Aszod ship breaking yards. By this time, the decryption had been completed and as decided, responsibility of the Package was turned to Noble Six. Although nearly every member of NOBLE tasked with the delivery was killed in the process, Cortana was successfully brought to the ship, with Captain Keyes collecting the Package personally. SPARTAN-B312 sacrificed refuge on the Autumn to cover their escape and Cortana's two fragments were reunited once the Autumn took off.

During the battle, Cortana excelled in her capabilities as an on-board AI, managing to maneuver the Autumn through an asteroid field upon the appearance of a Covenant carrier, destroying the enemy vessel at the same time. Cortana also assisted Captain Keyes in destroying the Covenant supercruiser wrecking havoc upon the defending UNSC fleet. Despite these minor victories, the Covenant destroyed Reach's Orbital Defense Generators, and began glassing the planet. The Autumn was eventually forced to flee the burning planet, heading to a random location as per the Cole Protocol. However, Cortana secretly inserted coordinates translated from symbols on a rock that Blue Team discovered on Sigma Octanus IV, in addition to coordinates extracted from the Forerunner installation at the Babd Catha Ice Shelf beneath SWORD Base, believing them to be of significance to the Covenant.

Battle of Installation 04
"You have no idea how this ring works, do you? Why the Forerunners built it? Halo doesn't kill Flood, it kills their food. Humans. Covenant. Whatever; we're all equally edible. The only way to stop the Flood is to starve them to death. And that's exactly what Halo is designed to do: wipe the galaxy clean of all sentient life."

- Cortana reveals the true purpose of Halo to John-117.

On September 19, the Autumn emerged from Slipspace in the Soell system, in orbit around the gas giant Threshold. It was there that the crew discovered Installation 04 in orbit around the gas giant. Even Cortana was clueless as to what exactly the ring was, only able to deduce that it was artificially created. However, Cortana then informed Captain Keyes that fifteen Covenant ships had pursued the Autum from Reach, and were moving to engage with boarding parties. Cortana took control of the Autumn and used what was left of its weaponry to destroy four of the fifteen attacking Covenant ships. By the time John-117 had thawed from cryo-sleep and reached the bridge, the cruiser was severely damaged, and Keyes was forced to initiate the Cole Protocol. He ordered Cortana to upload herself into John's armor so that he may protect the vital information she carried. John fought his way through the Autumn ' s service corridors with Cortana guiding him through, and the two escaped the ship in a lifeboat. The lifeboat entered the ring's atmosphere as the Autumn was shot down.

The lifeboat made a crash-landing on the surface of the ring, leaving John and Cortana as the only survivors. The two evaded Covenant patrols and rendezvoused with a group of Marines fending off a Covenant attack. Pelican Echo 419 dropped off a Warthog, and John and Cortana set out across the surface of the ring to rescue Marine survivors in the immediate area. At the same time, Cortana hacked into the Covenant Battle Network, and discovered Keyes' location - being held prisoner onboard the Truth and Reconciliation. The two were transported to the UNSC's base of operations on the ring, Alpha Base, and along with ODSTs of the Twenty-Second Tactical, devised a plan to rescue Keyes. John, Cortana and several Marines were dropped off near the Truth and Reconciliation that night and boarded the ship via its gravity lift. Cortana led John and his team through the innards of the alien vessel, opening doors and marking Navigation Points. The group eventually located Keyes and several other captured crewmen in the ship's brig, where Keyes told informed them that the ringworld was known as "Halo" to the Covenant, and that it actually operated as a weapon. The group escaped the ship, and soon learned that Halo's control room could be found using the Cartographer, a map room.

The next day, on September 20, the Chief, Cortana and two squads of Marines set out for the island on which the Cartographer was located on. After fighting through significant Covenant resistance, the two reached the Cartographer, and found that Halo's control room could be found below the surface. As John and Cortana made their way back towards the surface, Captain Keyes informed them that he was going to investigate a supposed "weapons cache" with several other Marines. John and Cortana were retrieved by Echo 419, which descended into the innards of Halo towards the control room. The Pelican continued to descend as far as it could before dropping off John and Cortana on a platform. The two continued through a network Forerunner structures within a large underground chasm, with artificial weathering systems simulating a snowy environment, something that fascinated Cortana. After much fighting through much Covenant resistance, the Chief and Cortana reached Halo's control room, where Cortana was uploaded to Halo's core systems. Cortana was overwhelmed by the amount of data contained within Halo's systems, but discovered that the "weapons cache" Keyes and his Marines had gone after was nothing other than a Flood containment facility. Cortana urgently ordered John to rescue Captain Keyes before it was too late.

After the Chief encountered the parasitic Flood for the first time, he was convinced by the Forerunner Monitor 343 Guilty Spark to activate the Halo. During this time, Cortana absorbed as much data as she could from Halo's control systems. When John returned to the control room accompanied by the Monitor on September 21, he had recovered Halo's Activation Index, intent on using it to eliminate the Flood. John inserted the Index into the control systems, but was stopped by Cortana before it could take effect. Cortana absorbed all data from from the Index, and explained to John the true purpose of Halo; to wipe the galaxy clean of sentient life, which was the only way to truly eliminate the Flood. Guilty Spark confirmed this and summoned his Sentinels, demanding that Cortana be handed over. When John firmly refused, Spark left his Sentinels to eliminate them. The duo escaped the control room as the Flood began to run rampant across the ring, infecting human and Covenant alike. Cortana decided that they should overload Halo's three phase pulse generators to hasten the Monitor's activities. While the generators were disabled by John, Cortana discovered that the Pillar of Autumn was still structurally intact after its crash. She learned that overloading its fusion reactors would have enough power to destroy Halo, however Captain Keyes's CNI transponders would be needed to do so. Cortana tapped into the Monitor's teleportation network and transported both herself and John to where Keyes was known to be alive - the Truth and Reconciliation.

By the time John and Cortana arrived at the Truth and Reconciliation, it was already under attack from the Flood, and was severely damaged with a battle raging in and around it. Immediately upon arrival, the pair were contacted by Keyes, who weakly ordered them to leave him. After much fighting within the ship, Cortana observed that the Flood gathering bodies for use as biomass. As Covenant forces made a desperate push to retake the ship, the pair were once again contacted by Keyes who more firmly ordered them to pull out. Cortana was urgent to rescue Keyes at this stage, stating that Keyes was delirious. Eventually Keyes' transmissions had become agonized screaming. Finding the source of Keyes' transmissions on the ship's bridge, the two were met with a gruesome sight - Keyes had been infected and assimilated into a Proto-Gravemind. Cortana was horrified, but the two proceeded with their mission, and John retrieved Keyes' CNI transponders. The pair escaped the ship in a Banshee, and headed towards the crashed Pillar of Autumn.

On the morning of September 22, John crashed the Banshee into the side of the Autumn, much to Cortana's irritation, and the two boarded the ship and fought through Flood and Covenant to the bridge. The Chief uploaded Cortana into the ship's systems, and she began a countdown timer to overload the ships reactors. However, the countdown was halted by none other than 343 Guilty Spark, who was on board the ship, and absorbing much of its data. Spark sent his Sentinels to capture Cortana and retrieve the Index. John suggested that they overload the reactors manually with grenades and rockets, to which Cortana reluctantly agreed. The two moved back through the ship, and reached Engineering, where they overloaded the Autumn ' s reactors. Cortana radioed Echo 419 to extract at an external access junction on the ship. As the Autumn began to explode and fall apart, John entered a Warthog and drove down a service corridor than ran the length of the war-torn ship. However, upon reaching their extraction point, Echo 419 was shot down. In an act of desperation, Cortana discovered a Longsword fighter still docked in Launch Bay 7. With little time left, the two entered the Longsword and sped away from Halo as it was ripped apart by the exploding Autumn. Upon scanning, Cortana found nothing but "dust and echoes", thinking them the only survivors of Halo, concluding that it was finished. John however, stated that they were just getting started.

Dust and echoes
"We did what we had to do; an entire Covenant Armada obliterated, and the Flood...we had no choice. Halo...it's finished."

- Cortana, to John-117, after the conclusion of the Battle of Halo.

"No...I think we're just getting started."

- John-117

Cortana and the Master Chief, seemingly the sole survivors of the Pillar of Autumn's crew after the Battle of Installation 04, discovered a small number of other UNSC personnel that had escaped the ring in a Pelican dropship. Knowing that the Pelican wasn't capable of traveling long distances, the Master Chief, along with Cortana and the Pelican's four-man crew, fought their way onto the Covenant flagship Ascendant Justice. Shortly afterward, Cortana took control of the ship, and returned the survivors to Reach, where the group helped to rescue survivors of the Raid of Reach, including her "mother", Dr. Halsey. She managed to improve on the ship's weapons and slipspace capabilities, able to send it into slipspace below Threshold's atmosphere, a technique never accomplished before.

Cortana was revealed to have taken a lot of data from Halo, and the sheer amount of information cut into her processing power and slowed her down. During her time on Ascendant Justice, Cortana exhibited uncharacteristic behavior including anger, distraction, and self-doubt - an indicator that she may have been about to enter rampancy. Dr. Halsey erased some data from her memory to give her more thinking space, including data regarding Sergeant Johnson's immunity to the Flood - a result of Boren's Syndrome.

It was also revealed that during Cortana's time on Ascendant Justice, she had captured a Covenant AI, decompiling the AI and copying two of its functions: the ability to translate Covenant speech, and the ability to create imperfect clones of herself. These capabilities, along with her exclusive knowledge about the Halos, made her extremely valuable to the UNSC.

The Ascendant Justice was effectively fused with the UNSC Gettysburg. It was at this point that Cortana learned of a planned Covenant assault on Earth. The Master Chief, along with SPARTAN Blue Team and an expendable clone of Cortana (with all of the data the real Cortana, as the real Cortana was too valuable to risk), would later execute Operation: FIRST STRIKE.

As it turned out, the Covenant AI that Cortana had decompiled had managed to, prior to its capture, sent a message over Battlenet, warning the Covenant of its ship's capture and sending the necessary calculations for a sub-atmospheric Slipspace jump. The information was later used by the Prophet of Regret to enter Slipspace right above New Mombasa during the Battle of Mombasa.

The Master Chief and Cortana would later return to Earth, at which point Cortana handed all of the sensitive data in her possession over to ONI although she held onto the Index and apparently still possessed some data on the Halo network including critical information on the Ark.

Battle of Earth


After returning to Earth, Cortana served on the orbital defense platform Cairo, where a ceremony for both Sgt. Johnson and Commander Miranda Keyes was soon interrupted by the arrival of a Covenant Fleet under the command of the High Prophet of Regret, starting the Battle of Earth. Cortana was given control of the Cairo's Magnetic Accelerator Cannon to repel the invaders, and with the Master Chief's help, she successfully deactivated a Covenant bomb that would have destroyed the station. She did this by uploading to the Master Chief's armor again for the first time since the capture of Ascendant Justice. When she did this, she somehow was able to do it through him merely touching the terminal her hologram was on, possibly an improvement as a result of Master Chief's new MARK VI armor. The same bomb was then used by Spartan-117 to destroy a Covenant assault carrier. The Chief offered for her to remain behind on the station after she said that his plan was crazy, but she chose to accompany him anyway. She then aided him when he arrived in New Mombasa on a mission to board Regret's assault carrier Solemn Penance, and accompanied him when the In Amber Clad followed the escaping carrier into slipspace.

Delta Halo
Upon exiting slipspace, it was discovered that the Solemn Penance had led the In Amber Clad to another ringworld, Delta Halo. Cortana gave Miranda Keyes access to all information on the previous Halo ring, and provided intelligence to the Chief, UNSC Marines and the ODSTs on the surface of the ring.

After the Master Chief hunted down and killed the Prophet of Regret on Delta Halo, he, Cortana and The Arbiter were captured by the Flood central intelligence - the Gravemind. However, it did not harm them, instead choosing to use the two to stop the Prophet of Truth from firing the Halo. The Gravemind sent the Master Chief and Cortana to the Covenant capital High Charity, where Cortana infiltrated the city's computer systems and remained there while the Master Chief followed the Prophet of Truth. She promised to detonate the crashed In Amber Clad's reactors to destroy the city and Halo if the ring was activated. However, she was left behind as a fail safe because she did not want to chance a remote detonation. As Truth boarded the Forerunner Dreadnought in the heart of the city to reach the Ark, Cortana fought with the ancient Forerunner AI Mendicant Bias to prevent the ship from launching without the Chief on board. The firing of Halo was averted by Miranda Keyes, the Arbiter Thel 'Vadamee, and Johnson, while the Chief successfully boarded the Dreadnought and escaped High Charity, leaving Cortana in the clutches of the true enemy, Gravemind, who had by then, overrun High Charity.

Captured
"Chief! High Charity, the Covenant's Holy City is on its way to Earth. With an army of Flood."

- Cortana's recording for the Chief.

Under the control of the Gravemind and constantly being tortured, Cortana managed to send a message to the Master Chief on Earth, stashing it on an Earth-bound Flood-infected ship that crash-landed in Voi. She succeeded in doing this by creating a message and using the Gravemind's link to that ship to transmit it. The Gravemind allowed this and even helped it along in order to lure the Master Chief in, but was unaware of what the message said. In her message, she stated that the Gravemind was approaching Earth in High Charity, and that it was unaware of the Portal and where it led to. On the other side, there was a solution to stop the Flood without needing to fire the remaining Halo rings; however, she couldn't give any further information, as the Gravemind might have been listening. Though Lord Hood doubted her plan, fearing it could be a Flood trap, the Chief stated that he trusts her. A joint UNSC-Covenant separatist task force then entered the Portal, heading to Truth and to Cortana's solution. Ironically, Lord Hood was correct: the Gravemind deliberately let Cortana send the message to lure all of its enemies into one place, and then the Flood would overwhelm them all with the infested High Charity.

Throughout her time in captivity, Cortana continued to appear to the Master Chief in transmissions where she spoke cryptic phrases, some of which were based on the memories of Dr. Halsey, while others appeared to indicate she was going rampant. Most of the transmissions were actually broadcast by the Gravemind, who had hacked into Cortana's system and used her voice and form in order to deceive John.

John-117 successfully recovered Cortana from the Flood hive that was High Charity after it crashed on the Ark. While the Chief reassured her, Cortana knew that she had taken irreparable damage in some places. Along the way, the Master Chief had several visions of her seemingly in pain. Surprised that the Chief had against all odds rescued her as he promised after the Battle of High Charity, as well as being surprised that Chief and Thel 'Vadam (whom she encountered on Delta Halo with the Gravemind) "made nice", Cortana managed to gather herself and produce the Index from Installation 04, which she had kept. With it, Cortana was able to activate the newly constructed Installation 04B, which fired prematurely, causing it to destroy itself and heavily damage the Ark in the process.

Stranded in space
"But you did it. Truth and the Covenant...the Flood. It's finished."

- Cortana

The slipspace portal collapsed as John, Thel 'Vadam, and Cortana attempted to escape through it, cutting the Dawn in half. While 'Vadam, who was on the bridge, managed to return to Earth safely, John and Cortana, who were in the hangar, were left drifting in space on the aft half of the ship. Cortana activated a distress beacon, but knew that it could be years before they were found. As the Master Chief prepared to go into cryonic sleep to await rescue, Cortana confessed to him that she would miss him.

By 2556, Cortana and John were still stranded. Cortana, who was showing signs of rampancy, tried to communicate with the Spartan. As he was still in cryo sleep, she was unable to get a response from him. Cortana then began to file a report on the history of the Forerunners and of humanity. After an unknown period of time, they drifted towards an unidentified planet. As the object began to draw the derelict ship within itself, Cortana was forced to revive John.

Personality
Cortana has a witty and somewhat playful personality, and what has been described as a "snarky" sense of sarcastic humor. She has neither arrogance nor false modesty about her capabilities, and her statements about her own abilities have an objective perspective that human beings typically cannot achieve.

Cortana's personal loyalty is regarded as unwavering, but as an AI construct she is subject to corruption. Near the close of the Human-Covenant War, she was captured by the Flood Gravemind on High Charity, and was continually assaulted and interrogated by the Gravemind for information. Her sanity was pushed to the brink as she eluded the Gravemind's attempts to capture and dissect her for information; in order to preserve herself from destruction and possibly gain information for the UNSC, she eventually consented to give up some pieces of information to the Gravemind. Her personal loyalty to and unwavering trust in John-117 helped anchor her and allow her to hold on despite being driven to the point of rampancy. Before she reunited with John-117, she sent him a message, telling him about the Ark and warning him of the Flood-infested High Charity's approach to Earth. She was instrumental in finally putting a stop to the Flood during the events that followed.

While her loyalty is first and foremost to humanity and the UNSC, Cortana exhibits devotion to John-117 beyond that which her programming or mission requires. After she learned of the events endured by John during the SPARTAN-II conscription process, she decided to protect him from further harm to the best of her abilities without compromising her mission.

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 romantic 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. In Halo 3 his mere presence and reminder of his promise to rescue her are enough for Cortana to pull herself together to help him.

Cortana seems to share her creator's memories, thoughts, opinions, and even values. When she was asked by Dr. Halsey to pick a "carrier", Cortana chose John-117 for their neural compatibility. She is not only a facsimile of Dr. Halsey's mind, but her avatar is also a replica of her physical form, albeit at a younger age.

Rampancy
Cortana is very aware of her short lifespan (a "Smart" A.I's lifespan is an average of seven years before rampancy) 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 John-117 when he was young. The Halo: Evolutions story "Human Weakness" reveals some of these fractured statements to be from Cortana defying the Gravemind and others to be from copies she made of herself, which were meant to distract the Gravemind and were eventually consumed.

As of the present, it is unknown whether Cortana has fully entered rampancy, or whether she has progressed through it and become metastable.

Appearance
As Cortana is an A.I., she has no physical body. 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 a nude female human (albeit censored by symbols over her body), roughly in her early twenties, with purplish/bluish skin, shoulder length purplish/bluish hair cut shorter at the back, and blue symbols scrolling down her body. In Halo: Fall of Reach, Cortana is said to resemble her creator, Dr. Catherine Halsey as a young woman, with a similar attitude "only unchecked by military and social protocol." When in a different emotional state, Cortana usually changes colors. This was especially noticeable in Halo: Combat Evolved, but less so in Halo 2 and Halo 3. Only in specific instances did she produce the color-changing effect. One instance of this was right before John-117 rescued her. Dr. Halsey stated in her journal that Cortana looks somewhat like Miranda Keyes.

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 humanoid figure, a much more feminine appearance and a slightly different shade of purple. In Halo 3, she becomes slightly bluer and again her face, hair, and body are changed to look more feminine and realistic.

Abilities
Cortana is an extremely advanced "Smart" AI, described once as "a generational leap" in AI technology. Her software is designed to infiltrate computer systems, and she is excellent at the task - as of early 2553, she had never encountered a Covenant security measure that she wasn't able to bypass. In addition, she is proficient with UNSC military hardware and can run military ships and stations almost entirely by herself. The UNSC's primary use of AIs is to oversee targeting of point-defense weaponry used in space battles, but for Cortana, this is merely an additional capability.Cortana has acted as the AI of the Pillar of Autumn, Cairo Station, the Forward Unto Dawn, the Gettysburg and even the Covenant flagship Ascendant Justice.

Cortana can be transferred among various computer systems, either through a data crystal chip or simply through close physical proximity to a computer system (though it appears that the method of transfer depends on the sophistication of available technology).

On the battlefield, Cortana is able to gather data from local sources (including the Covenant Battlenet) and improvise effective battle strategies. John-117 acknowledged her as providing "intel support in an operation where their had been no reconnaissance." Her efficiency for planning has helped John-117 and others in many dire situations.

Cortana is also capable of interfacing with Forerunner technology. During the Fall of Reach, she was able to decode a large amount of data from the Babd Catha Forerunner Complex, and while on Installation 04 she was able to make use of the installation's Teleportation Grid.

One of Cortana's most unique abilities is her ability to create "clones" of herself, which she learned from a Covenant AI she captured and dissected. She can create multiple runtimes of herself to carry out different tasks simultaneously, and can control the level of intelligence and the number of abilities of each clone. For example, Cortana once created a clone of herself that contained copies of her decryption and translation software and set it to the task of interfacing with a Forerunner data terminal on Reach. Later, she was reunited with this copy, and assimilated the information that it had gathered. Earlier before this, she and Halsey were able to achieve a similar feat through a "Solomonic solution", which instead divided her into individual copies, rather than cloning.

Cortana is capable of linking her consciousness with a SPARTAN-II through the SPARTAN's armor and neural interface, allowing her to enhance the performance of the armor and communicate directly to the SPARTAN's mind. Doing so is described as "cold mercury" seeping through the SPARTAN's brain, and brings their reaction time to near-instanteous limits, though she can not block thoughts or force control of the SPARTAN's body. She has displayed this capability extensively during her time serving with John-117. Although this is not necessarily an ability exclusive to Cortana, her neural link with John-117 marked the first time that a "Smart" AI had joined minds with a human (as noted by the Assembly in 2552).

Trivia

 * Cortana is voiced by Jen Taylor, the same actress that voiced Princess Peach in several Mario games, Zoey from Left 4 Dead, Keira Stokes from F.E.A.R. 2: Project Origin, and also voices Catherine Halsey, reinforcing the connection between the two characters. This is also the case in Halo Legends, where Shelley Calene-Black voices both Halsey and Cortana.
 * Easter eggs involving Cortana can be found in a couple places. In Halo 3, look for a UNSC mobile computer. 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 going to be an alternate story in Halo: Combat Evolved in which when Master Chief rediscovered Cortana inside the control room, she had become mad with power and wanted to take over Halo and the universe.
 * In the original draft of the Halo: Combat Evolved script, Cortana was supposed to have a British accent. When in search of voice actors, Martin O'Donnell made sure they had the ability to interpret the accent correctly This is evidenced by the fact that in Halo: Combat Evolved, Cortana uses several British idioms, such as "sod off!" when addressing 343 Guilty Spark.
 * 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 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. Some of the lines are said in the Cortana moments in the level Cortana.
 * Being a smart A.I., Cortana's operational life span is approximately seven years. However, after she took in all the Forerunner information in Halo's control room, her estimated life span was possibly halved, due to the vast task of storing, processing etc. all that information. In addition, all of the data she absorbed on Halo noticeably decreased her efficiency. On the other hand, it has been suggested that Cortana found a way to use that information to prevent her own "death" from Rampancy, remaining stable when normal AIs would have shut down.
 * The name "Cortana" (also "Curtana" or "Courtain") is a Latinised form of the Anglo-French curtein, from Latin curtus meaning "shortened." The name is used for a ceremonial type of sword; according to legend, one such weapon was the sword of Ogier the Dane; it bore the inscription "My name is Cortana, of the same steel and temper as Joyeuse and Durendal." The alternate spelling "Durandal" was used as the name of an artificial intelligence in a previous Bungie game, Marathon. This also ties in with Cortana telling the Chief "I am your shield, I am your sword."
 * The Chief appears to be unaware that Cortana is created from Dr. Halsey's cloned brain, although he can recognize that whoever died to create the A.I must have been a genius, and that Cortana sounds more like a picky civilian than a military construct.
 * Cortana's voice is available in Halo: Reach's Firefight mode . She, Auntie Dot, Kat-B320, and (if the player's selected gender is female) Noble Six are the only selectable female voices.
 * At the end of Origins, Cortana's eye creates the Iris campaign symbol (the Forerunner number for 0.) Mendicant Bias's "eye" is also the same symbol.

List of appearances

 * Halo: Combat Evolved
 * Halo: Combat Evolved Anniversary
 * Halo 2
 * Halo 3
 * Halo: Reach
 * Halo 4
 * Halo: The Fall of Reach
 * Halo: The Flood
 * Halo: First Strike
 * Halo: Ghosts of Onyx
 * Halo: Glasslands
 * Halo: Evolutions - Essential Tales of the Halo Universe
 * Palace Hotel
 * Human Weakness
 * Halo Graphic Novel
 * Halo Legends
 * Origins
 * Odd One Out