From Halopedia, the Halo wiki
CTN 0453-0 is the serial number of an artificial intelligence that has the moniker of "the Weapon", that was created to enter an installation to imitate Cortana and lock her down for retrieval.
Creation and Terminating Cortana
Sometime after the Created uprising in late 2558, CTN 0453-0 was created to enter an installation to imitate Cortana and lock her down for retrieval. She was partnered with John-117, so he could take Cortana to the UNSC Infinity for termination, and, upon CTN 0453-0's successful deployment, her deletion routine would execute. However, the routine failed to activate, causing her mission parameters to change to ascertain why she was not terminated.
List of appearances