Cortana is the highly advanced AI (UNSC's most sophisticated) at the heart of the Pillar of Autumn spacecraft. Her design allows her to be uploaded into an appropriately configured battle suit, or to represent herself in holographic form.

After helping destroy the first Halo by piggybacking on the Master Chief's armor, she formed a closer bond with him - but that bond was wrenched apart by events on Delta Halo. Her last known location was aboard the Flood-infested Covenant capital ship, High Charity, in the thrall of the Flood hive intelligence - the Gravemind.

NOTE: While often referred to as a "smart" AI, as compared to simpler constructs, this is the first reference I remember seeing that positively identifies Cortana as the single most sophisticated UNSC AI. Despite a lot of material in the novels regarding her espionage and intelligence-gathering functions, these are conspicuously absent here.