Vergil

Vergil was a subroutine built into the larger Superintendent Artificial Intelligence, later merged with a renegade Covenant Huragok.

Biography
Vergil was created by Dr. Endesha to look after his daughter, Sadie Endesha. Using the seemingly endless resources of New and Old Mombasa, Vergil would monitor her through the Superintendent's network of security cameras and microphones. It was even capable of overriding municipally-controlled machines when it believed it was acting in Sadie's best interest. For example, it brought a public train to a halt in an attempt to prevent Sadie from moving towards New Mombasa when her intention was to sign up with the UNSC, which Vergil believed would put her in unnecessary danger. Vergil performed its tasks without fault, although Sadie found it to be a little restrictive. It isn't quite clear whether or not Vergil was truly sentient, or merely an incredibly advanced computer program that simulated the reactions of a sentient mind very well.

Battle of Mombasa
When the Covenant Invasion of Earth and the attack on Mombasa took place, Vergil was instrumental in Sadie's escape from harm in her struggle to reach her trapped father, commandeering numerous vehicles to assist her, as well as the defense of the city by local militia, police, and UNSC Marines.

In his attempts to find Sadie, Commissioner Kinsler shut down the Superintendent, in order to prevent it from helping her. Eventually reactivated, Vergil had been badly damaged and was near to total system shutdown, saved only by the efforts of a renegade Covenant Huragok. Unwilling to continue as slaves to the Covenant, seven Huragok had made an attempt at escape. Six of them were destroyed during this attempt by giving their lives so that the seventh could flee. Eventually, it made its way to the core of the Superintendent, working to repair it. As it worked, the Huragok began to merge the badly damaged Vergil with itself, a process that was complete just as it was discovered by Captain Veronica Dare and the Rookie. Early in this process, Vergil, now stored within a Covenant Huragok, helped to free Sadie from Kinsler by opening the doors of his private train and letting the mob of refugees rip Kinsler apart.

Later, the new Vergil would defect to the UNSC, allowing itself to be captured by Veronica Dare and her team of ODSTs, to provide the UNSC vital information on the Covenant, and their intentions for New Mombasa. A month later, Sergeant Major Johnson had a talk with Vergil, who was in the process of accessing the Covenant Battlenet, about the information it had. Vergil agreed to share any information it has with the Sergeant and lit his cigar as a friendly gesture.

Trivia

 * The name Vergil itself may be a reference to the Divine Comedy, specifically to Virgil, the poet that guides fellow poet Dante through the nine circles of hell and purgatory, much in the same way the Rookie is guided through a metaphorical hell-on-earth, a destroyed New Mombasa, and also through the nine "circles" of audio logs, a reference to the nine circles of hell.
 * Like the other Engineers found in the game and in spite of its lack of armor, Vergil still has shields to protect himself, and also is able to grant close friendly units shields, including the player. However, the player has no shield monitor, and if playing on Legendary, the shields decrease in strength rapidly.