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 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 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". 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.

Trivia

 * 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 universe.
 * 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.
 * Alternatively, Didact could have simply asked the Monitors for advice using FTL communications, and activated all the Halos at once from the Ark.