If your character left the service under amicable circumstances and you’d like for him to retain “Courtesy” rank, you may want to consider investing a couple of points in Rank.
Born to the Harliston family, Corbin was expected to follow in the long standing family tradition of joining the Navy. His father’s fathers all the way back to Richard Haliston in 1425, Vice Admiral of the British Navy, had always joined the Navy and advanced to positions of rank and privilege. Corbin instead decided to go with his friends and join the Marines. He stuck with the Marine air corps to at least stay with the family tradition of piloting craft, but his family refused to accept his choice and completely disowned him. This would not have been so bad if it wasn’t the fact that his family has been in positions of influence and power throughout the military. He routinely found himself being passed over for promotion and placed in the worst assignments mankind could have. Of course no matter his postings he always managed to receive commendations from his immediate commanders. This finally led him to the highest rank he could attain in the field with no future for advancement and the worst possible job posting he could get, a landside job. He and his talents were wasting away planetside and he had come to the conclusion that it was time to start looking at getting a civilian job when an unknown organization approached him. He was going to join D.R.A.G.O.N. with an unspecified job, unspecified pay and barely specified objectives. At least it was time in the cockpit again.