Ancilla

Ancillas are a type of Forerunner artificial intelligence construct. They come in several varieties, each designed for different purposes and possessing differing capabilities. Known models include the Monitors, the Contender-class AIs, and the personal ancillas carried by most Forerunners.

Background
Ancillas can be stored within a Forerunner's body armor and can be accessed by the user at will. They can also be housed within personal shells like those of the Monitors, as well as ships, buildings, and various forms of machinery. They typically possess personalities approximating that of a Forerunner; in this respect, they are quite similar to human "Smart" AIs.

The most powerful ancilla, next to the Contender-class, is the Metarch-class ancilla which were used exclusively by the Forerunner Council. When a Forerunners is a Manipular, the ancilla tries harder to make itself more "visible". But once the Forerunner mutates once, the ancilla makes itself less visible so it can divert that power to other tasks; making it much harder for the user to access them. It is also apparent that the ancilla help the Forerunner access the Domain after mutation.

Personality
Ancillas, although they each of separate personalities and minds, also seem to work together. For example, one ancilla may change its personality if it helps the Forerunner, by accessing the old ancilla's database and reprogramming itself with the data given by the old ancilla. But the best thing that helps Forerunner and ancilla work together is the ancilla's ability to read the user's mind. The ancilla, if allowed by the Forerunner, will access the Forerunner's personality and reprogram itself to be the most compatible with the Forerunner.

Trivia

 * The Forerunners' relationship with their ancillas is strikingly similar to that of John-117 and Cortana. Like Cortana, Bornstellar describes his ancilla as a "blue female figure in the back of [their] mind[s]" who can be stored within their body armors, and relies upon them for guidance and help in many situations.
 * "Ancilla" is Latin for 'female servant' or 'maid'.
 * So far, we have not seen any personal, Male Ancillas.

List of appearances

 * Halo: The Flood (2010)
 * The Forerunner Saga
 * Halo: Cryptum