A spy of some sorts might be, if the other players didn't mind someone on the team working against them in a way. Well, feeding information isn't going to hurt to much, just ruin a couple of plans.

The original idea was an otherworlder who didn't want to go home. For whatever reason, they liked Earth more. Maybe they were a second-class citizen there, or part of a philosophy that opposed the war... A double agent's loyalty can be tested during their service, after all.