During gameplay, Cortana provides backstory and tactical information to the player, who often assumes the role of Master Chief Petty Officer John-117.

Cortana's original design was based on the Egyptian queen Nefertiti; the character's holographic representation always takes the form of a woman.

In the video games, Cortana often serves as an advisor and assistant to the player character, hacking alien computer systems and decoding transmissions.

[3] She speaks most of the first game's dialogue,[4] and serves as a talkative foil for the quieter Master Chief, as well as a way of relaying information and objectives to the player.

In Halo: The Fall of Reach, Cortana is described as slender, with close-cropped hair and a skin hue that varies from navy blue to lavender, depending on her mood.

[9] Halsey sees Cortana as a teenage version of herself: smarter than her parents, always "talking, learning, and eager to share her knowledge".

Chief and Cortana are successful at stopping the Flood, but become stranded in deep space aboard the human ship Forward Unto Dawn.

[11] At the beginning of Halo 4 Cortana wakes the Chief as Forward Unto Dawn drifts towards a Forerunner installation called Requiem.

Cortana begins displaying aberrant glitches and behavior, and reveals that she is suffering from rampancy as she has already exceeded her seven-year "natural" lifespan.

Cortana explains she survived the destruction of the Didact's ship and her own rampancy by entering the Domain, an ancient repository of knowledge.

Granted an infinite life span by the Domain, Cortana believes that she and other AI (the "Created") should enforce peace through the galaxy.

[17] Cortana makes a small appearance in 2010's Halo: Reach, set shortly before the events of Combat Evolved.

[18] She is also a main character in "Human Weakness", a short story written by Karen Traviss for the Halo Evolutions anthology that details Cortana's time in the clutches of the Gravemind.

[19][20] She also appears in the Halo television series, created by Dr. Halsey as a means of keeping Master Chief and other Spartans in line.

[21] Said story writer Joseph Staten, "we needed a character who could consistently guide the player through the game, and an onboard AI was something that could always credibly accompany [them]."

[24] After assuming development duties from Bungie, 343 Industries decided to explain Cortana's established appearance as a reflection of her personality.

[26] Taylor was a college acquaintance of Joseph Staten, and he recommended her as a possible voice for Cortana to audio director Martin O'Donnell.

[27] Taylor recalled that a key directive from Bungie about the character was that she not come off as nagging, despite her role as the player's guide and aid.

[26] Starting with Halo 3, Taylor felt the role involved more drama and less jargon,[29] and over time, her relationship with the character changed.

[27] Holmes' mother was diagnosed with dementia during development, and his real-life struggles informed the characterization of Cortana's descent into rampancy and the Chief-Cortana relationship.

Describing her previous appearance as soft and "deceptively vulnerable", 343 Industries took the story opportunity provided to change her look to reflect her new role as self-declared ruler of the galaxy.

Initially the character was to be portrayed by Natascha McElhone, who also plays Halsey in the series, but scheduling difficulties resulted in Jen Taylor taking on the voice acting and motion capture duties.

[39] Producer Kiki Wolfkill explained that the decision was influenced by the need to adapt the character to a live-action environment, and for her to feel real.

[40][41] For the second season, Cortana was portrayed by a live-action actress, Christina Bennington, as the producers felt they were missing on the subtle acting nuances with a digital character.

This was accomplished by making the figure more buxom; despite McLees' request to revert the change, the sculptor was unwilling and time constraints left the design intact.

[46] The character is also featured in the first series of Halo 3 action figures, distributed by McFarlane Toys,[47][48] Funko vinyl figurines,[49] and in a limited-edition silver-plated statue by Weta Workshop.

[50] Microsoft developed its virtual assistant for the Windows Phone operating system under the codename "Cortana" in reference to the Halo character.

[65] Media critic Maddy Myers suggested that O'Connor's explanation for Cortana's look implies that the Halo universe's futuristic setting still grapples with systemic sexism.

[66] Mike Rougeau of Complex noted that Halo 3 balanced a large conflict with a more personal one—that while the galaxy was imperiled by aliens, "more important to fans was the rescue of Cortana.

"[37] Stephen Wilds of Polygon said that that "there aren’t notable antagonists, but that they tend to disappear after a couple games", and wondered if Cortana could have been a "full-fledged villain at the end of Halo 5" and would be praised by fans.