Regina is the only playable character. She is excellent at stealth actions, and in customizing weapons.
Gail is your team leader, and a bit of a hard ass. He prizes nothing more highly than the completion of a mission, and will do so at any cost.
This guy is basically the team's hacker. If something computer-related needs to be done, Rick is the man to do it. Unlike Gail, Rick cares more about his comrades than any mission.
Cooper is your communications officer, which means he carries your radio equipment. Unfortunatley, he misses the landing zone and is eaten by a Tyranosaurus Rex in the opening scenes of the game.