Sliced from my previous post: [s]EDIT:

Actually, the idea just popped up. Perhaps during MAS training, the KRISS & Atlas entity didn't want to see its inventions go to waste, so they offered affinity tests to trainees and Erwin exhibited the highest cohesion with the MAS and its fighting style? Since his personality bears a resemblance to a mouse, which correlates to how the Zeitgeist operates, could be possible.[/s]

Well, I could go either way to be honest. Although a rich kid can easily be edited into his backstory, I'm aware another rookie doesn't really seem too good for the plot, though that's not to say he would react like the other rookie after his first skirmish, nor would he have to be babysat. But since the farm life taught him morals and to be modest, guess I'll figure out a way to part with the rookie archetype.