Sif

Sif was an UNSC A.I. on the Tiara Space Station, in orbit above Harvest. Near the center of the Tiara was a room which housed her data center that contained the processor clusters and storage arrays that served as her core logic.

Avatar
Her chosen avatar takes the form of a woman with an ankle-length sleeveless gown and long golden hair; she also gives the appearance of Nordic royalty. She chose this avatar because many of Harvest's settlers where from Earth's American heartland, which could trace their ancestry back to Scandinavia.

Role
She is in charge of Harvest's shipping operations which included the balancing of the Tiara, the transporting of human and shipping traffic up and down the space elevator, and control of the passenger and cargo transport in and out of the Tiara. The A.I. Mack flirts with Sif often, usually to her annoyance and frustration. Deep down however, Sif loves Mack.

Right before the tiara was destroyed, Sif, after being destroyed by Loki, was revived by the Huragok Lighter Than Some and she went rampant. She died right after she told Mack that she loved him, but it is unknown if her message got through to him, although it is known that Mack loved Sif. Soon after, Loki shot a mass driver round into her data center, destroying her and the Tiara.

Trivia

 * Sif is the wife of Thor in Norse mythology.
 * In Norse mythology, Sif and Loki factually did not correlate with one another often. In the Myth, The Treasures of the Gods, Loki manages to get into Sif's locked bedroom and hacks off all of her golden hair as an unfeasible prank. Thor eventually caught him and made him retrieve hair from the Dwarves. Like in Halo: Contact Harvest, Sif is constantly irritated with Mack (Loki's) presence, comparable to their Norse mythology predecessors.
 * Sif was one of the few AI that went rampant, ultimately ending in her destruction.