Cortana (voiced by Jen Taylor) is an Artificial Intelligence construct created by Dr. Catherine Halsey (The creator of the Spartan-IIs). She is the Master Chief's companion throughout most of the Halo series. Cortana is a "Smart AI" meaning she has the ability to learn and adapt, as opposed to a "dumb" AI, which has only one purpose- point defense of a cruiser, for example. However, due to their complexities, "Smart AI's" have an operational life of seven years, after which they literally think themselves to death. Cortana was created from a flash-clone of Dr. Halsey's brain, meaning she thinks similarly, and may share physical similarities with the Doctor when she was younger, including appearance and voice.
Cortana was created shortly before the fall of Reach, and was specifically designed to work with the SPARTAN-II soldiers on a mission to capture a Covenant Prophet. The MJOLNIR Mark V suit was specifically built to interface with an AI such as Cortana, which further improved the Spartan's combat efficiency. Cortana was allowed her choice of Spartan soldier to interface with, and she chose the Master Chief out of thirty other candidates, as referenced in the opening cinema of Halo 3. The Spartans were to disable a Covenant ship, have Cortana fly it back to the Covenant homeworld, where they would capture a Prophet and use it to attempt to broker a peace agreement. However, this mission went awry with the unexpected arrival of Covenant forces at Reach just as the task force was about to get underway. The Master Chief handed Cortana over to Captain Keyes to assist in the Naval battle, while the Spartans went groundside, which is why Cortana first appears as the shipboard AI on the UNSC Pillar of Autumn.
After the battle at Reach was lost, the Pillar of Autumn jumped out-system on a semi-random vector. They emerged from slipspace near Halo, where Cortana destroyed four Covenant frigates before surrendering control of the ship to Captain Keyes and heading down to Halo with the Chief. Cortana assisted the Master Chief in his efforts to rescue Captain Keyes and find the control center to the Halo ring. After arriving in the control center, Cortana realizes that the Flood had been released, and sends the Chief alone to try and stop it before it spreads. While the Chief is in the Library with 343 Guilty Spark, Cortana absorbs a huge amount of information from Halo's control center, which she later passes on to the UNSC.
The Master Chief retrieves her, whereupon she reveals the true nature of the Halo installations. Cortana takes the Index to the Halo array to prevent 343 Guilty Spark from firing it. However, she realizes that he may find another way to activate it, and the only true way to be sure is to destroy Halo. The two succeed in detonating the Pillar of Autumn's fusion reactors, and escape the ring in a longsword fighter.
After the discovery of Delta Halo, Master Chief and Cortana hunt down and kill the Prophet of Regret. Fleeing from an orbital strike from Brute carriers, the duo encounter the Flood Gravemind for the first time. The Gravemind transports the chief and Cortana into High Charity, where they fight their way through a Covenant civil war to the Forerunner Dreadnought in the center of the city. The Chief boards the ship, headed back to Earth, while Cortana stays behind, so she can make sure that Halo does not fire. In a short scene after the Halo 2 ending credits, Cortana is seen conversing with the Gravemind.
The Brute and Elite fleets put aside their differences for a moment and attempt to quarantine the flood escaping Delta Halo, one infected carrier escapes, headed towards Earth, and with a message from Cortana aboard it. From the message, it is clear that Cortana is becoming corrupted, as she appears in pain and does not always speak coherently. It tells the humans to head through the recently opened portal, and that there is a solution to the Flood beyond it. The portal leads to the Ark, beyond the limits of known space, and certainly out of the firing range of the Halo rings. Shortly after the human and elite forces arrive to fight the brutes, so do the Flood: an infected High Charity jumps into the atmosphere and comes crashing down upon the Ark.
After discovering a rebuilt Alpha Halo to replace the one the Master Chief destroyed in Halo: Combat Evolved, the Master Chief sets out for High Charity to reclaim Cortana, who appears in cryptic messages throughout the level in various states of distress, even exhibiting some signs of Rampancy (See: Marathon). After the Master Chief retrieves her, she is shaken, but not too badly damaged. She reveals that she still has the activation index from the first Halo ring, which they can use to arm the rebuilt ring above the Ark. The Gravemind, who has been holding Cortana hostage, is enraged by this revelation, as the ring could very well kill the Gravemind and the entire Flood outbreak if activated.
The Chief and Cortana activate the ring, then escape the installation on a Warthog with the Arbiter. They make it to the UNSC frigate Forward Unto Dawn by jumping into the hangar bay with their warthog. They speed off to the Portal at full speed, as the Halo primes to fire behind them. Unsure if they will make it or not, Cortana says: "It was a pleasure serving with you John", as the screen fades to white. This is the only time in the Halo trilogy that anyone ever calls the Master Chief by his real name.
After the credits, a brief stinger reveals the Chief and Cortana, in the rear section of the Forward Unto Dawn, which has been sliced in half by the portal. Cortana sends out a distress beacon, but says it could be years before anyone even finds it. The Chief climbs into a cryo tube, and tells Cortana to "Wake me when you need me". In the Legendary ending, the ship can be seen drifting towards what looks like a planet with a blue nebula glowing over the horizon. The planet was later revealed to be Requiem, the planet that Halo 4 takes place on.
Cortana wakes Master Chief as Covenant forces begin attacking the Forward until Dawn then crashes with him on to Requiem. During the game Cortana's behaviour becomes more erratic exhibiting graphical and vocal glitches, showing signs of rampancy, which is occuring as she has operated far past her predicted and designed 7 year lifespan. The chief protects her from the captain of the Infinity who wishes to delete her and she helps to defeat The Didact. She uses her rampancy against the rogue forerunner, fragmenting her personality and infecting systems with it.
Finally, she protects the chief from the destruction of the Composer and uses the last of her energy to manifest as a solid hologram, bidding him farewell.
Halo 5: Guardians
In Halo 5: Guardians the Master Chief and his allied Spartan-IIs track down Cortana. They find her on the Forerunner world of Genesis, protected by an aggressive warrior calling himself the Warden Eternal. Blue Team overcome the Warden and the Forerunne's other defences and meanwhile learn that Cortana has become enamoured with the old Forerunner plan of using oppressive methods to attempt to bring peace to human worlds. She also claims to have cured Rampancy. Cortana refuses to return home with the Master Chief and instead causes an uprising among the AIs possessed by humanity, hundreds of who join her cause.
As the series has progressed, Cortana's appearance has changed noticeably, from being a noticeable purple in Halo: Combat Evolved, to a bluer colour with a different hairstyle in Halo 2, to being absolutely blue and far more defined in, Halo 3. Her appearance in Halo 4 has not changed much. The most noticeable change is that her hair is darker and her previously solid feet now end in distinct toes. She only makes a cameo appearance in Halo: Reach and only has one line, repeating her very first line of dialogue from Halo: CE. The same audio file that was used in the game and in its remake, Halo: Combat Evolved Anniversary. In Halo 5 Cortana initially appears as she had done in the preceding game but is later shown to have adopted a layer of armour the same blue colour as the rest of her.
Microsoft developed a digital assistant for their windows phone to rival Apple's Siri. They codenamed the assistant Cortana and retained the name for final product release when it was seen to be very popular.
Jen Taylor also provides the voice of Microsoft Cortana.