Although AIs are not rare in the age of the Halo series, Cortana is a special kind of AI: a smart AI. Smart AI's are created from cloned brains, giving them the ability to learn. Cortana was created from a clone of Dr. Halsey's brain, which is how Cortana got her distinct personality and disregard for UNSC protocol. However, as the smart AI's learn more, they can think less, giving them an average lifespan of seven years. Cortana has acquired many large masses of information over her adventures with Master Chief, and has acted erradically on occasion, although it's unclear whether or not Cortana has gone rampant.

As the Pillar of Autumn is being invaded by the Covenant in Halo: Combat Evolved, Cortana escapes in Master Chief's armor in accordance with Cole Protocol to prevent the Covenant from getting data that may lead them to Earth. Cortana is being held by Gravemind at the end of Halo 2, and her last line in the game has led people to speculate whether or not she will help Gravemind.