Hey, I'm glad you liked the idea enough to run with it! That makes me very happy. My idea was that he/she had operated outside of the normal scope that most see of a Sentinel. My character, Genevieve, does manipulating and coercion, but she is overt. So my thought was for a covert, almost soviet era spy-like character.
I could see this being something along the lines of him/her distinguishing themselves from their contemporaries through exemplary service and Auron decided to grant them full Sentinel status after accomplishing something amazing. This could easily be made up. Like, assassinating a high profile Alpha Alliance member or whatnot. After being granted Sentinel status, he could be assigned with the others and make an appearance where we are now.
Just some basic thoughts I had running through my head. Hope it helps! :3