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.

He was constantly trying to convince the librarian to give up on her mission to save other sentiant life from the halos and return to him "where it is safe. His pleas, however, were unsuccessful and the Librarian ended up self-distructing her fleet and stranding herself on earth to live out the rest of her days in a place she refered to as "eden"

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.