Loki

Loki was an Planetary Security Artificial Intelligence construct in service with the United Nations Space Command. It was stationed aboard the Phoenix-class Colony Ship, until it landed on Harvest. Loki and Mack actually shared the same data center and processor; however, only one could use it at any one time. Mack showed Sif that in his data center, he had two matrices (it is assumed that one is for Mack and the other is for Loki). When the swap was needed, Mack would send the relevant data to Loki's matrix before Loki assumed control of the data center (and by extension everything Mack had influence over on Harvest). Besides a few minor variations of the avatar, mainly the eye color (Mack's eyes were gray, Loki's were red) and cleanliness (Loki's avatar was clean whereas Mack's was dirty from "working in the field"), they looked the same.

When the Tiara was boarded by Covenant forces, Sif sent a distress signal intended for Mack. However, by that point Loki had already switched places with Mack. Mack had told Loki to keep Sif safe; however, Loki believed it would be better to destroy Sif to protect all other human colonies. He unleashed a powerful virus upon Sif that purged her code throughout all her clusters save one, which he kept operational to monitor the invaders. When Lighter Than Some began repairing the Tiara's data center, he found Loki's fragment and they developed a makeshift plan to lure the Covenant warship Rapid Conversion in range of the planet's mass driver. Loki succeeded in executing the plan, disabling the warship and commencing the evacuation of Harvest's surviving population. However, he hadn't anticipated a counterassault by the Covenant, and was saved only by Mack, who was inhabiting the JOTUNs. Mack then turned on Loki, using his JOTUN "all-in-ones" to hack the maser dish and destroy Loki's fragment in the Tiara (his attempt to protect what little was left of Sif). After the evacuation was complete, Loki sought to finish the job he had started (destroying Sif) but was once again faced with stiff resistance from Mack's army of JOTUNs. However, Mack did concede to Sif's agreement that her destruction was the best thing for humanity, and Loki fired a mass driver round directly into her data center, completely obliterating her. The full details of the nature of Loki's own destruction are not known; Mack later does reveal that Loki was destroyed in an explosion that consumed Harvest's reactor (which housed Mack/Loki's data center).

Trivia

 * Loki is named after the god of chaos in Norse Mythology. He was an adept shape-shifter and could change not only his form, but also his sex, possibly referencing the ability to switch between himself and Mack. In Norse Mythology, he usually plays the role of villain and trickster.
 * There is a group of structures called "The Pillars of Loki", which could be a reference to him.
 * Loki seems to be the only AI with two different egos; himself and Mack.