Ur-Didact

"Mendicant Bias is trying to prevent us from firing the Array. He speeds back to the Ark, but he won't succeed. Offensive Bias will stop him, and I will burn this stinking menace in your name. And then? I will begin our Great Journey without you, carrying this bitter record. Those who came after will know what we bought with this [false transcendence] - what you bought, and the price you paid."

- The Didact's final transmission.

The Didact was an individual who held a high status in the Forerunner Military. He evidently wholeheartedly believed in the "Mantle" the Forerunners held to protect life. He was also the lover of the Librarian, and the one who eventually activated the Halo Array.

He was constantly trying to convince the Librarian to give up on her mission to save other sentient life from the Halos, and return to him "Where it is safe". His pleas, however, were unsuccessful, and the Librarian ended up destroying her own fleet, stranding herself on Earth to live out the rest of her days in a place she referred to as "Eden". Didact evidently died when he activated the Halo Array as a last resort to stop the Flood.

Trivia

 * A conversation between Didact and the Librarian 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 fired.
 * In the Iris campaign you can read Didact's final words right before the Halo array Fires. When they fire he stops talking, indicating that he died from the Array.
 * The word "Didact" actually means "one who teaches" in Latin. From this information, one could infer that at one point in his life he did some important teaching in Forerunner culture. The name Didact could come from the word didactic which means to instruct others in morals. This is rather ironic because he wound up being the one committing galactic suicide, firing the metaphorical gun at the head of the galaxy.
 * From 343 Guilty Spark's dialogue in the level Two Betrayals, it can be assumed that the Didact activated the Array by first lighting Installation 04, allowing the other rings to automatically follow suit.