Bill Tyson was a staff sergeant assigned to the marine garrison on Mars under Master Sergeant Kelly. He was, amongst other things, responsible for training new marine arrivals. In the months leading up to the demon invasion, Sergeant Tyson noticed that morale amongst the troops was beginning to drop due to a number of incidents occurring at the Mars City. He suspected (correctly) that the mysterious UAC experiments were the cause of these events. In response, Tyson suggested that squads be rotated every 90 days instead of every 180 days. He also doubled security at all checkpoints.
Tyson was one of the many people killed or possessed when the demons invaded. His body, along with his PDA, can be found be found in Mars City just outside Marine HQ. The player can use his PDA to learn the access code for the weapons locker at Marine Command.