Little is known about the Captain prior to the events of Left Behind, except that she served in the 4th Infantry Division as part of the crew of a helicopter. During her birthday, the unit celebrated, one gifting her a picture of the group together, playfully drawing a party hat on her head, and calling her "the coolest captain around".
When sent out on a rescue mission, Regan and her unit crash-landed in the Colorado Mountain Plaza after their patient turned. The infection was presumably via spores, as none of the crew saw any bites. The crew's pilot, Sean Brendon, crashed the helicopter after being attacked by the patient, and presumably died from the impact of the crash. Larry Caulfield became infected when fighting the infected patient, although he managed to save the lives of the rest of the crew. Regen deemed him a hero for having saved them during the crash and from the Infected. However, following protocol, Regan had Private Ellis subdue Larry and proceeded to execute him, something she later regretted. It is unknown how long they were trapped in the plaza, but they were there long enough that they began to run low on supplies.
Regan and Private Ellis began to search for food but were overrun by Infected. Although they escaped, Ellis was bitten. He tried to commit suicide, but Regan stopped him, knocking him out and amputating his arm in an effort to save him, breaking protocol.
Over the next few weeks the pair desperately attempted to survive. Ellis seemingly survived the infection, only suffering from a fever, but Regan still suspected he was infected. Their relationship was strained by the lack of food, Ellis even assaulting Regan at one point. Regan even considered abandoning her friend, and actually made it 10 miles from the mall, but remembered that her unit had become her family and decided to stay.
However, Ellis became delusional and thought Regan was planning to kill him. While talking, Regen saw Infected and went for her gun only for Ellis to leap at and beat her to death in fear she was going to kill him. Ellis was quickly attacked by the infected, re-opening his wound. He bled to death in an air duct, shortly after recording himself saying he was sorry for killing Regan and wishing she hadn't gone for her gun. 
Events of Left Behind Edit
Ellie finds Regan's helicopter and proceeds to take a medical kit that was inside to use to save Joel's life. Ellie is also able to find the artifacts left by each of the soldiers, and their corpses. She finds Regan's in a small room next to the crashed helicopter, still holding the photo of her with her crew.