The UNSC AI companion who accompanies Master Chief in Halo: Campaign Evolved is Cortana.
Cortana is an artificial-intelligence construct created by the United Nations Space Command. She serves as the Master Chief’s mission partner, providing tactical analysis, navigation, intelligence, and access to systems that would otherwise be unavailable to him. Her relationship with the Chief is one of the defining partnerships in the Halo story.
In the original Halo: Combat Evolved campaign, Cortana travels with the Chief after the UNSC ship Pillar of Autumn reaches the Halo ring. Together, they investigate the installation, fight the Covenant, and uncover a threat far more dangerous than they initially expected.
The Weapon is a different AI introduced later in Halo Infinite, while Isabel and Serina are associated with other Halo stories and settings. Cortana’s importance extends well beyond one game: she appears throughout major parts of the franchise and becomes central to its political and technological conflicts.