Well, it being a point that TFA was his first official mission and that he hadn't gotten into any real trouble before that movie does limit the story opportunities in the pre-TFA era. Probably doesn't help that the different sources can't agree if he was a completely unremarkable, run of the mill trooper or the top of his class prior to his defection (the latter is more commonly acknowledged, but the former tends to fit the characterization more commonly used).