Sam Harper is an officer in the U.S Army.
He's a proud member of the elite Army Rangers.
He is definitely not a spy and does not want to be and if given a choice would almost certainly bow out of anything remotely related to the sort of activities he now routinely gets himself volunteered for. Volunteered, you see, by Michelle. [Note: no doubt Michelle has a last name but nowhere in any of the recounts of his adventures do I find it.]
Who exactly is this powerful Michelle who has such control over him? Well, at times he refers to her as a friend but other times, and far more often, he says she is his ex-girlfriend. If we take that latter on face value, it would seem that she and he are no longer an item but the influence she has over him is pretty impressive. And she does not do it through wheedling really so much as just gentle, subtle but effective persuasion.
And why does this Michelle want to get him involved in all the clandestine activities that we watch the two of them risking their necks in? Just her being an old acquaintance from his high school days would not be enough. Did I mention she is an agent with the CIA?
Harper is first and foremost an electrical and mechanical engineer and a darn good one at building interesting doo-dads. He got to college and became an officer via ROTC but would have likely served as an engineer instead of the Rangers except that Michelle wanted him to work with her on a little project she and the Agency had going in Korea (North, wouldn't you know) and to do that he had to be with the Ranger Military Intelligence Platoon so that meant he had to become a Ranger. It was tough but so was he (except for dealing with Michelle, apparently).
And that is how a smart surfing-loving 1LT Sam Harper gets involved in a lot of CIA activities.