Ur-Didact

The Didact, was the lover of the Librarian; these two beings were 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. He held some high status in the Forerunner Military, due to hints in the Terminals, and evidently wholeheartedly believes in the "Mantle" the Forerunners hold to protect life. He is also the one who activated the Halo Array.

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.