To make money and become the biggest crime bosses in the region. It's a small region, so it would be easy to control. If they control the Pokemon, and the power, then they can control the region. You know, like control towns and businesses and things like that.
Heck, we could have a three man party, or something.
Also, you don't start out as Apprentice, as far as I know. Thats just the characters the tutorial gives you. It's recommended by the character creation screen as well, but that's just a suggestion. You can choose any of the classes.
I would like to take a moment and selfishly extend a request for you to try out the Squire class.