Guess I was a bit discreet in the biography. Basically he had to infiltrate for some of his ops and they had to send someone that would appear to have a good reason to be on the ship other than that. His combat record gave him that ability to mask his mission, and since only the IOA rep and Commander (Maye a couple others) Would have access to his records (And that's only if they were American). Basically he's a spy in the way that his true motives aren't known, but he's there to sniff out and deal with other spies if possible. Like you noted though he has no specific training in this field so the way he does this job goes back to his official assignment of protecting the crew.

-Edit- And If I start making no sense I'm doing a lot at once so I apologize!