Warning: Contains spoilers for Halo episode 3!
Halo has changed Cortana's origin and purpose in six major ways. Introduced in 2001's Halo: Combat Evolved, Cortana was a sophisticated artificial intelligence who bonded with the Master Chief's Mjolnir armor. She served as his adviser throughout his missions against the Covenant, although in sequels Cortana sadly suffered rampancy, actually becoming an antagonist in her own right. The Halo TV series is, however, taking a very different approach to the original Halo timeline when it comes to Cortana - making her as much a threat from the beginning as an ally.
The first three episodes of Paramount+'s Halo TV series essentially serve as Cortana's origin story. The actual process is lifted straight from the lore of the original game universe, where Cortana was originally created using illegal flash-cloning of Spartan creator Dr. Catherine Halsey. Halsey then used a process called Cognitive Impression Modelling to create a scan of the flash-clone's brain; this fashions an artificial intelligence-based on the so-called "donor" brain, but unfortunately destroys the original.
But, while the methods Dr. Halsey used to create Cortana may be surprisingly accurate to the show's source material, a lot of the details are very different. Paramount+'s Halo series has added greater depth to Cortana's backstory, giving her a sinister new role and abilities that seem to be subtly different to the original timeline. Here are all the major changes to Cortana's origin and purpose.
Cortana Seems To Have Been The First Smart AI
Smart AIs are extremely common in the main Halo timeline. It's not unusual to see major UNSC frigates manned by smart AIs, and many ran planetary defense systems. Even the main timeline's version of the Rubble, an Insurrectionist headquarters concealed within an asteroid system, was controlled by a smart AI; it carefully controlled the drift of countless asteroids, ensuring the Rubble's main habitats remained viable. In contrast, in the Halo TV series Cortana appears to have been the first of her kind. Dr. Halsey believed the creation of Cortana was literally the next step in human evolution, transforming a human mind into a virtual form for the first time.
Dr. Halsey Created Cortana To Control Master Chief
Surprisingly, Dr. Halsey's purpose in creating Cortana appears to have been far more sinister than in the main timeline. There, Halsey simply believed the AI would be an essential assistant in unraveling the true motives of the Covenant; in contrast, in the Halo TV series she views Cortana as the next step in the Spartan program. Cortana has not been designed to serve as a mere assistant, but rather to merge with John-117's consciousness. This would vastly amplify the speed of the Spartan's thoughts - he'd be able to make detailed tactical assessments at speed, interact with technology around him at will, and respond to stimuli human beings cannot detect - but it would do so at a cost. John-117's mind would essentially be absorbed into Cortana, meaning he would essentially cease to exist as an individual being. This, Dr. Halsey concluded, would make John-117 perfectly controllable. Dialogue in Halo episode 2 strongly hints Cortana is just the first of her kind, with Dr. Catherine Halsey intending to create duplicates to possess the bodies of the other 30+ Spartans.
Cortana Didn't Name Herself - Dr. Halsey Did
Cortana named herself in the main Halo timeline, choosing a name that is a variant of Curtana - the sword used by the legendary French hero Ogier the Dane. Such names are actually something of a tradition for Halo's creators Bungie, because the AI in their previous game Marathon 2: Durandai was also named after a legendary blade. But in the Halo TV series, Cortana does not name herself; Dr. Halsey has chosen that name for her. This may seem to be a subtle difference, but it's an important one. It's the difference between Cortana having a sense of independence from the start, and (at least initially) being bound to Dr. Halsey's will. As seen in Halo episode 3, she starts out spying on the Master Chief on Dr. Halsey's behalf, as perfectly controllable as her creator intended. Fortunately for John-117, his own brain patterns are somehow required to activate the Forerunner artifact, meaning Dr. Halsey can't afford to continue with this process just yet.
Cortana Manifests As A Full-Size Hologram From The Beginning
In another notable difference, the Cortana of Paramount+'s Halo TV series manifests as a full-size hologram from the moment of her creation. There's nothing canon-breaking about this, because Cortana's body is holographic, limited only by the hologram-generators in the vicinity. But it's still rather strange seeing a full-size Cortana strolling around chatting to the Master Chief, rather than the hand-held hologram seen in the games. It's possible Cortana will revert to a smaller size during combat situations in areas without holographic technology, where she'll be dependent on whatever holo-generators are installed in the Master Chief's Mjolnir armor.
The Cortana Chip Is Implanted In The Master Chief's Brain
In the Halo games, the Cortana chip is implanted in - and interacts with - the Mjolnir armor. There's still a connection between Cortana and the Master Chief's mind, because he controls the Mjolnir armor through cybernetic fibers bound into his own neural and nervous systems; but it's indirect. In contrast, in the Halo TV series Cortana is implanted directly into the Master Chief's brain, meaning she cannot be removed. This gives her the potential to expand her consciousness into John-117's own mind, as Dr. Halsey intends. It will be interesting to see whether Cortana stays there over the course of Halo season 1, or whether John-117 forces her to be removed.
Cortana's New Origin Gives Her Very Different Abilities
This new origin gives Cortana very different abilities to those in the game. In Halo: Combat Evolved, the Master Chief is able to unplug the Cortana chip from his Mjolnir armor and plug her into Forerunner technology on the first Halo Ring; this, of course, won't be possible in the TV show timeline. While Cortana can certainly interact with technology around her, accessing and even hacking into the most sophisticated data systems, she is bound to the Master Chief's physical presence. This is actually a dramatic limitation on Cortana's abilities, one the Master Chief may come to regret as the story of Halo continues.