The AI companion introduced in Halo Infinite is called The Weapon.
The Weapon was created by Dr. Catherine Halsey as a copy of Cortana, designed specifically to imitate and contain the original AI. Her intended mission was to infiltrate the systems connected to Cortana, lock her down, and then self-delete. However, events aboard the UNSC Infinity and Zeta Halo leave The Weapon active and partnered with Master Chief.
During Halo Infinite, The Weapon gradually learns about Cortana’s actions and develops an identity separate from her template. Master Chief initially treats her cautiously because of the danger posed by Cortana’s earlier rebellion, but their partnership becomes central to the campaign.
The Weapon is not simply another name for Cortana. She resembles Cortana because she was based on the same source, yet the story presents her as a distinct artificial intelligence with her own decisions and personality.