Lt. Williams is the pilot of the Bloodhound dropship in charge of giving the squad their objectives. His image is displayed on the left side of the user's screen during his messages. He dropped off the squad to start their mission and later picks them up at the end of Timor Station. He isn't in charge of the marines' mission (though it may seem so at first because of his rank), he is just sent along with them. He also offers the marines assistance in Deima Surface Bridge, lighting the bridge with "Bloodhound's" external spotlight, shooting the swarm with side mounted guns, and finally using the ship as a bridge for marines to cross a chasm.
UT 2k4 Bio
The original Lieutenant Williams first appeared in the second mission of the "Sigma" campaign. There he is described as "a commanding officer attached to a forward scout unit". During the mission he has to be recovered, because he "holds vital information regarding the campaign area and Swarm movements."
His second appearance is in the second mission of the "Escape Form Rhea" Campaign, where you have to "meet Lt. Williams and his team at the southern spillway and proceed together to the extraction point."