Nah, we could have them.

Though, they don't necessarily fight. They'd be more like the aforementioned "civilian" classes, which would probably not partake in a battle, but for example, would remain at the allied camp to raise spirits and morale after a bloody battle so they can be motivated for the next battle, in a war sense. Just an example, though.

[@Rakurai]
I think the gist of having a trainee class is kind of obsolete, as well. I intend to have it that you could introduce a character in Part I, and have them as active characters for the rest of the Parts, which could potentially constitute their growth from "trainee" to "veteran".

Altogether, I think the term "trainee" would just mean inexperienced as whatever class they are, and not defined as an individual class.