An avatar is the visual representation of an artificial intelligence or "AI", used as a way to better interact with humans. Avatars often appear as human-sized wherever sufficient projection equipment is available. With more limited projection systems such as a starship's holo-pedestal, they appear considerably smaller.
While not used by all AIs, every "smart" AI uses an avatar as do many "dumb" AIs. This visual representation is chosen by the AI upon their creation and can be whatever they desire it to be. The creation of an AI's avatar occurs very early in the system's birthing process, and unless an AI is programmed to have multiple avatars, it does not usually alter its appearance after its initial creation, and attempts to do so have resulted in catastrophic AI system crashes.
Although the duty which the AI is to perform, combined with the human brain that was used to create it, have a great deal to do with what avatar they chose to appear as. Without exception, AI systems identify themselves as male or female based on the gender of their brain donor. In addition to their visual representation, AIs are also able to choose their preferred accent or style of speech.
 List of known UNSC AIs and their avatars
- Auntie Dot: A grid of blue-glowing, constantly changing symmetrical patterns.
- Black-Box: A featureless blue cube. Has been known to alter his avatar on occasion; he once appeared with a bright red bow tied around him in recognition of Captain Serin Osman's birthday. Uses a battered, worn version of his avatar to represent his "dumb" processes.
- Captain Teach: Appearance and mannerisms of an archetypal pirate captain, with gold teeth and black hair, carrying multiple pistols.
- Cortana: Tall and slender woman, nude but censored by scrolling calculations and symbols across her body, with short-cut hair in Halo: Combat Evolved, then long hair in Halo 2 and Halo 3. She strongly resembles her brain donor, Dr. Catherine Elizabeth Halsey. She glows bright blue.
- Déjà: The appearance of a Greek goddess: barefoot, wrapped in a toga, motes of light dancing about her hair and holding a clay tablet in her left hand.
- Endless Summer: A tall, red Cherokee Indian chief, bare-chested, wearing buckskins, and holding a feathered spear.
- Iona: One of several known AI with more than one appearance, her default appearance is that of a slender woman with short, curly hair with angular shapes across her body, reminiscent of a skin-tight suit. She also appears to wear boots and knee pads. She has an additional appearance resembling a child version of her default avatar for her more simplistic "ghost" interface to deceive remote scanners. She glows dark red.
- Lysithea: A sweet feminine voice with a laugh that sounds like "fine bone china clinking together."
- Rebecca: She is one of the few known AI to have two appearances. Her first appearance is described as "half-Athena, half-Ares" with a feathered Greek headdress and armor. She can change her appearance to a flabby, middle-aged Mediterranean woman in a flower dress.
- Roland: A yellow man wearing an aviator outfit. More specifically the WWII British fighter/test pilot Roland Beamont.
- Serina: A tall and slender woman with long brown hair, wearing black boots, tight pants or leggings and a white shirt. She glows bright blue.
- Sif: A slender woman with a long neck, elevated chin, and long blonde hair, wearing a multicolored, ankle-length sleeveless gown. 
- Wellsley: Stern-looking man with mildly long hair, prominent nose, and a collared coat; supposed to look like an 18th century British officer. Is said to resemble his namesake, Arthur Wellesley, the Duke of Wellington.
 Unidentified AIs
There were also at least three AIs at the Damascus Testing Facility on Chi Ceti IV that were have not been specifically identified, with the avatars of a samurai, a mermaid, and a figure made entirely of bright light with comets trailing in her wake. In addition, there were two unidentified AIs on the UNSC Point of No Return, one of which was a wraith-like gray-robed figure without a body, and the other a collection of disembodied eyes, mouths, and gesturing hands, noted by Kurt-051 as resembling cubist art.
- ^ Halo: The Fall of Reach, page 38
- ^ a b Halo Wars: Genesis
- ^ Halo: Fall of Reach - Boot Camp
- ^ Halo: Combat Evolved, campaign level The Pillar of Autumn
- ^ Halo 2, campaign level Cairo Station
- ^ a b Halo Encyclopedia, page 220
- ^ a b Halo: The Fall of Reach, page 28
- ^ a b Halo: Contact Harvest, page 30
- ^ Halo: First Strike, page 127
- ^ Halo: The Fall of Reach, page 95
- ^ Halo: Glasslands, page 45
- ^ Halo: Evolutions, "Pariah", page 50
- ^ Halo: Ghosts of Onyx, page 106
- ^ Halo: Ghosts of Onyx, page 178
- ^ Halo: Blood Line - Issue 1
- ^ Halo: Ghosts of Onyx, page 147
- ^ Halo: First Strike, page 100
- ^ Halo: Evolutions, "The Impossible Life and the Possible Death of Preston J. Cole", page 422
- ^ Halo: Contact Harvest, pages 28-29
- ^ Halo: Evolutions, "Midnight in the Heart of Midlothian", page 83
- ^ Halo: Evolutions, "The Mona Lisa", pages 231 and 233
- ^ Halo: Evolutions, "The Impossible Life and the Possible Death of Preston J. Cole", pages 463-465
- ^ Halo Wars
- ^ Halo: Contact Harvest, page 68
- ^ Halo 3: ODST
- ^ Halo: Evolutions, "The Impossible Life and the Possible Death of Preston J. Cole", pages 448-252
- ^ Halo: The Flood, page 209
- ^ Halo: The Fall of Reach, page 114
- ^ Halo: Ghosts of Onyx, page 60