Twenty-six years into his military career, Captain Keyes commands respect as a keen strategist and inspirational leader of his men. He became a minor hero early in his career, when he led a small group of security troops against a Covenant ambush of the colony ship Meriwether Lewis and held them off long enough for the ship to escape. His many decorations and years of combat experience against the Covenant made him a natural choice to command the Pillar of Autumn and its secret cargo.
Captain Keyes doesn't directly affect the actual blood-and-sweat combat of the game in any way, but he's a central character nonetheless. As the Captain of the Pillar of Autumn
(not to mention highest ranking officer in god-knows-how-many thousands of light years), he's the man to talk to about your orders. Plus, you've got to rescue his sorry ass more than once. You'll get to know Mr. Keyes very well over the course of the game.