Cortana, CTN 0452-9

Cortana has a witty and playful personality, with a sarcastic sense of humor. She originally had a deep friendship and bond with several Spartans, each having saved the other's life many times throughout the Covenant war. While she is not boastful, she does take pride in her adaptability as an AI, and occasionally teases those around her about her invaluable talents.

Cortana seems to share some of her creator's memories, thoughts, and opinions, for instance favouring the SPARTAN-IIs even at her birth as an AI. 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. However, Cortana was torn over the morality of the SPARTAN-II project, vowing to protect her Spartan from any future threats. This was decisively demonstrated when Cortana set out to ruin the career and reputation of Colonel James Ackerson for his attempted killing of John during a training exercise.

When Reach was being attacked by an insurmountable force mustered by the Covenant, Cortana had chosen Noble Six, a SPARTAN-III of Noble team to ferry her to the Chief on the pillar of Autumn, all of noble team save for Jun-A266 (who split off to ferry Halsy) and Noble Six-B312, who would finish the mission as his final duty.

While her loyalty was initially first and foremost to humanity and the UNSC, Cortana exhibited a unique devotion to beyond that which her programming or mission requires and eventually ended up feeling more loyal towards those she served with. After she learned of the events endured during the SPARTAN-II conscription process, she decided to protect them from further harm to the best of her abilities without compromising her mission. During the confrontation with the Didact, Cortana was so loyal to the Chief that even her rampant parts worked together with her to save him, telling the Didact that they were not doing it for humanity, implying she was doing it for the Chief. She ended up essentially sacrificing herself to ensure the Chief survived the destruction of Mantle's Approach.

Cortana's personal loyalty is regarded as unwavering, but as an AI construct she is subject to corruption and rampancy. 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. Her stability was pushed to the brink as she eluded the Gravemind's attempts to capture and dissect her for information. Among the Gravemind's favorite tactics were exploiting Cortana's insatiable desire for knowledge, the nature of her artificial existence, and providing her glimpses of thoughts and feelings from humans absorbed into the Flood collective. 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 to her sanity. She was instrumental in finally putting a stop to the Flood during the events that followed.