Tai Dai,
Thank you for your excellent reply! That gives me a much better knowledge of how to write the bio.
Regarding your last paragraph, if we assume the character gets in because the DCMS is in need of MechWarriors quickly instead of due to political connections or extreme wealth, does he apply to enter the academy, or does he get automatically sent there if his aptitude tests results are excellent? Asked another way, does he get to attend because he wanted to (and he's good enough), or because whoever is in charge of these things saw his test scores and decided that it would be a waste not to have him at the academy?
You're welcome!
Even in 3145 you'd still need some wealth/connections, just less so on the spectrum. Whatever increased opportunities may become available due to expanding the Army would mostly manifest well below Sun Zhang's level. Going back to the difference between ostensible truth and truth in practice: I'm sure there's some panel at Sun Zhang that looks at applications for admission and judges who makes the tiny cut. How much or little corruption goes on is set by your headcanon for the universe :) But even if those officers are above ever taking bribes, there's issues like whos application ever even shows up on the panel's docket to be reviewed in the first place. Similarly, there's probably problems with getting access to even be tested for mechwarrior aptitude in the first place. Remember High Schools don't exist in the Combine. If you're not wealthy/connected enough for college prep school, you're off into the workforce after elementary school. Child labor isn't just legal, it's the law!
That being said, wanting to go to SZ isn't quite the same thing as wanting to enlist in the DCMS. Most would-be servants of the Dragon with non-noble backgrounds would end up in the infantry or other conventional forces. Becoming a Mechwarrior is a fairly big deal akin to becoming a Knight in medieval Europe. If you're from a poor family, your odds of being successfully recruited into the Mechwarrior corps are vastly better by buying some time at a regional DCMS proving ground than out-competing some noble brat for a seat at a good academy. However, PCs are indeed not "most people" and special backgrounds are the norm :D
With regards to such a background story: I'm sure there'd be some sort of Combine-wide standard test that your character presumably did great on to even begin the process of getting someone to sponsor you to SZ. The story hooks that need to be plausibly satisfied are "where'd the wealth come from" or "why did someone with the clout to do so bother to sponsor you in". Off the top of my head, perhaps your character's ancestor did your sponsor's family a solid. Perhaps your sponsor wants to prove that his backwater fief isn't so petty since someone from "his" lands got into SZ. Perhaps your character was the happy benefactor from some vendetta between petty noble Houses (Your sponsor got you in to SZ rather because it denied a seat to his Rival's son). Etc.