The Weapon

"The Weapon" is the moniker of an artificial intelligence, she has the serial number of CTN 0453-0. She was created to enter an installation to imitate Cortana and lock her down for retrieval. The Weapon is actually an exact copy of Cortana herself, with all of the same routines and systems but with any information that could compromise her removed.

Creation and Terminating Cortana
In October 2559, Dr. Catherine Halsey dispatches Blue Team on Operation: WOLFE to retrieve three cryobins and a lockbox from a cryovault beneath the ruins of CASTLE Base. Halsey insists that what they are retrieving is the key to stopping Cortana although she warns John-117 that while it won't be easy, it is the only option that they have. The contents of the bins, the remaining cloned brains from Halsey's creation of Cortana, are used by her to create the Weapon whose mission is to enter an installation to imitate Cortana and lock her down for retrieval. Halsey creates the Weapon to be an exact copy of Cortana with any information that could compromise the Weapon removed by Halsey. According to the Master Chief, she is basically Cortana if the two of them had never met.

After her completion, she is partnered with the Master Chief, so he could take Cortana to the for termination, and, upon the Weapon's successful deployment, her deletion routine would execute. However, while she succeeds in locking Cortana down, the routine fails to activate, causing her mission parameters to change to ascertain why she was not terminated.

Zeta Halo conflict
The Chief successfully deploys the weapon on Zeta Halo and, assuming the Weapon has deleted itself according to plan, returns to the UNSC Infinity. However, the Weapon's deletion did not go through as planned, and she was left active in the Foundation, thinking the chief was due to come back for her. However, The Banished would launch a four-pronged attack on the Infinity, crippling it, forcing an full evacuation and ultimately a crash on the surface of Zeta Halo. Master chief would be left drifting in space after being thrown off infinity's launch bays by The Banished Leader, Atriox.

Six months after the defeat of Cortana, the Master Chief is revived and returns to the foundation, where he is surprised to see the Weapon is still functioning. Not knowing how and why her deletion was averted, she is recovered by him, and goes on to accompany him across Zeta Halo, now controlled by The Banished following the crippling of the Infinity. Recovering the framework that the Weapon had used to lock down Cortana, the two witness echoes of Cortana's life scattared across Zeta Halo's systems. While interfacing with Zeta Halo at the Command Spire, she is momentarily trapped in the system by The Harbinger, who has been monitoring her to learn how to work the ring's systems. Master Chief, worrying the AI has been compromised, goes through the authentication to manually delete the Weapon, but through yet-undetermined means, she overrides the process herself. The Weapon is livid that Master Chief had a killswitch for her this entire time, and that he was willing to use it. She would continue to work with him, but her tone shifted to outright dejection and sarcasm towards the Chief. The echoes across Zeta Halo would eventually lead the Weapon to discover her true nature as a copy of Cortana, who then decides to allow the chief to go through with her deletion for fear of going power mad herself. However, the Chief refuses, admitting that while he doesn't trust the AI completely, he wants to.

The Weapon eventually deduces her deletion was prevented by Cortana Herself, after reviewing echoes Cortana had deliberately left in the Silent Auditorium for them, as a form of archive. They recorded Cortana's inner thoughts and her exchange with Atriox following his attack on the Infinity, all which amounted to her realizing she unwittingly set in motion a chain of events which will put Master Chief (whom she correctly assumed was not dead, despite Atriox' backed claims to the contrary) in mortal danger, along with everyone else in the galaxy. Knowing human forces on Zeta Halo were (at best) on the back foot, she would sacrifice herself to destroy the Silent Auditorium, Giving them time to regroup, and hopefully find and revive John-117. She also knew The Weapon would be John's sole option for stopping the plans of The Banished, as she would no longer be able to. Presumably, she had not only overrode the deletion, but transferred the sequence over to The Weapon's matrix, allowing her to reject John's attempt to manually delete her at the command spire.

After the defeat of the Harbinger, the two witness a final message recorded for the Master Chief by Cortana who apologizes for her actions and hopes that the two learn from her mistakes and become better for them. Cortana also tells the Master Chief that the Weapon has a lot of potential. After reuniting with Echo 216's pilot, the two learn his real name and the Master Chief offers to let the Weapon choose her own name, much like Halsey had done for Cortana. The Weapon informs her companions that she has the perfect name for herself.

Personality and traits
The Weapon has a sunny and naive personality due to her young age and lack of experience. Due to Halsey apparently limiting the information that the Weapon has access to, she lacks basic knowledge on things such as what the Banished and even the ex-Covenant species such as the Hunters are. However, she shares some traits with Cortana such as often snapping her fingers when performing an action and, due to possessing the same routines and systems as Cortana, she possesses Cortana's skill in hacking into alien systems and retrieving or sabotaging things. The Weapon sometimes does this with enough of a dramatic pause before finishing that that the Master Chief asks her if something's wrong before the Weapon snaps her fingers and completes what she was doing.

After discovering her true origin, the Weapon is horrified because of the terrible things that Cortana, who is essentially her, has done and she tries to delete herself. However, the Chief stops her, stating that while he doesn't trust her, he wants to, recognizing the differences between Cortana and her copy. Cortana herself notes that the Weapon has a lot of potential and that she makes a great replacement as the Master Chief's AI companion.

Trivia

 * The Master Chief's deletion code for the Weapon is "RED FLAG 034 SAMUEL." Operation: RED FLAG is the mission to the Covenant homeworld High Charity that the Master Chief and Cortana were supposed to embark upon when they first met while Samuel-034 was his best friend before Sam died in the Battle of Chi Ceti in 2525.
 * In Halo Infinite, the Weapon first transfers into the data crystal chip by touching it in the same way that Cortana did in Halo 3 before transferring into other systems by the Master Chief touching them like Cortana did in Halo 2, albeit with the Master Chief first manifesting a hologram of her above his hand.
 * In Halo 4, Cortana briefly predicts the future partnership of the Master Chief and the Weapon at the end of the level Composer, telling the Master Chief that "they'll pair you with another AI. Maybe even another Cortana model if Halsey lets them. It won't be me, you know that, right?" In her final message to the Master Chief, Cortana jokingly echoes this statement to him.

List of appearances

 * Halo Infinite