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, and evidently wholeheartedly believes in the "Mantle" the Forerunners hold to protect life.

Trivia

The word "didact" means "one who teaches." From this, one could infer that at one point in his life he did some important teaching.