Ur-Didact

The Didact was the lover of the Librarian; these two beings were presumably Forerunners and a conversation between the two can be found inside the Terminals in Halo 3. When the Terminals are accessed, the player is eventually moved to their old conversation that was recorded before the Halo rings were ever fired. At one point the Librarian tells the Didact "You've outwitted it, my love," suggesting that the pair are not related, as few members of the same family would call each other "my love," (however, it is unknown how Forerunners communicate with each other).

He/She is believed to hold some status in the Forerunner Military, due to hints in the Terminals.

It is possible that the Didact is an AI construct commissioned by the Forerunners, as the Librarian and Didact repeatedly discuss the limits of their "Mantle". Whether or not the Didact is an AI depends on what described the battle in the Dyson sphere in which the Halo effect was fired; as all biological life forms were destroyed by the effect, it can be assumed that the one fighting for the Forerunners was artificial in nature.