The Monitors
The Monitors are highly advanced Artificial intelligence constructs created by the Forerunners over one hundred thousand years ago to occupy special positions in Forerunner society. One of the main purposes of a Monitor was to be the caretakers of the Halo Array as well as other installations(such as the maintenance of Line Installations) and to ensure that the virulent Flood remain imprisoned. Monitors are extremely intelligent, yet completely devoted to their original function, and are zealous about containing Flood; Monitors have been known to violently turn on their allies if they should attempt to violate their containment protocols.
The Covenant refer to the Monitors as Oracles. Individual humans appear to use whatever names they deem appropriate; Sergeant Major Avery Johnson has referred to the Monitor 343 Guilty Spark as "light bulb", "tinker bell", and "robot", and John-117 simply shortened his name to "Spark".
The body of a Monitor consists of a roughly spherical shape, concave on three sides, with an illuminated photoreceptor (an eye) located on the "front" of the orb. A Monitor's silvery metal covering is reminiscent of other Forerunner constructs, though their spherical shapes are a sharp contrast to the polygonal, angular shapes commonly seen in other Forerunner creations. The two Monitors seen so far speak in a tinny, but level, male voice.
Monitors' eyes are branded with the Marathon symbol and tend to have a variable hue; 343 Guilty Spark's eye color is blue, though it has changed to red during moments of what appear to be rampancy; 2401 Penitent Tangent's eye color is perpetually red despite no evidence of combat-related behavior other than his proximity to a Gravemind form. Whether eye colors vary between Monitors, between their action- or emotional states, or both, is unknown. During the SDCC 2011: Halo Universe Panel, a small video made by the "Sequence Group," shows Monitors that each have their own color.
Each Halo Installation has at least one Monitor assigned to it, with Installation 05 having two: 2401 Penitent Tangent, and a large Monitor on a monorail, seen in Halo 3's Cold Storage.
Monitors have both numbers (presumably for identification) and names. A Monitor's identification number appears to be 7 raised to the power of the Monitor's Installation number minus one. The names appear to consist of an adjective describing a negative emotion followed by a technological term, though neither of the two names seen appear to have any specific meaning. In Forerunner communication logs, Monitors are identified by their Installation number (padded to two digits), a dash, and their own number; for example, 343 Guilty Spark, Monitor of Installation 04, is identified on such logs as "04-343."
Each Monitor commands the Constructors, Sentinels, Sentinel Majors, and Enforcers of the Installation they monitor. The latter two are only activated if a catastrophic outbreak occurs, to hold back the Flood, while the Monitor locates a Reclaimer.
Monitors are extremely intelligent, but are also devoted to their original functions and to protocol, resulting in (among other things) a zealous attitude toward containment of the Flood and an overeagerness to activate the Halos. Monitors frequently cite protocols as explanations for their actions, no matter how impractical; this has been known to confuse or annoy modern-day Humans who have interacted with them. It is possible that a Monitor sees their protocols as the only options in a situation; in Halo: Combat Evolved, 343 Guilty Spark could have taken numerous precautions to prevent the Flood from spreading across the Installation, but the only action he ever considered was the activation of the Halo.
Nearly every action taken by a Monitor is based on some sort of protocol; even their greetings appear to be standardized. Of the two Monitors seen in the Halo series, both have introduced themselves as follows: "Greetings. I am [Number] [Name]. I am the Monitor of Installation [Number]." The only variation seen is the occasional reversal of the latter two sentences.
Though a Monitor's knowledge of its own Installation is seemingly limitless, Monitors' knowledge is otherwise quite limited. In particular, 343 Guilty Spark does not appear to be able to distinguish between different Reclaimers until the events of Halo 2, and the construct has admitted to a lack of knowledge about other Installations, and it can be assumed that the Forerunners practiced this compartmentalization of information with the other Monitors as well.
Monitors keep daily logs of all things that occur on their Installation. As with UNSC AI, the Monitors have been speculated to be in stages of Rampancy -- a condition that affects human "Smart" AIs and is comparable to insanity—due to their isolation for literally a hundred thousand years.
Though not a monitor the human smart AI "Cortana" has absorbed so much Forerunner and Covenant software and information that she has been able to go toe to toe with the more advanced artificial intelligences and survive unscathed.
Cortana, UNSC Artificial intelligence serial number CTN 0452-9, is a "smart" A.I. construct, or "Intelligent Agent". She was one of the most important figures in the Human-Covenant war, and served as John-117's partner in various combat missions, as well as serving as the A.I. for the United Nations Space Command Halcyon-class cruiser UNSC Pillar of Autumn and the Orbital Defense Platform Cairo Station. In addition, she held vital data pertaining to the Halos, including the Activation Index from Installation 04.