Overview & Background
343 Guilty Spark is the caretaker of Installation 04, and serves as the primary antagonist of Halo: Combat Evolved and Singleplayer Version 3. He is introduced at the end of the mission 343 Guilty Spark, where he convinces John-117 to activate Installation 04, in order to stop the Flood from spreading.
However, when John-117 attempts to activate the ring, it is revealed by Cortana that the Halo array's purpose is to wipe out all sentient life in the galaxy in order to starve the Flood of its food supply. In light of this discovery, John and Cortana set out to destroy the ring, rather than activate it.