Another avenue is to be the children of landed nobles whose holdings protect the border. Perhaps something like a margrave?

It would be both politically and strategically damaging, if the lord responsible for holding the border was missing his oldest and brightest children on the eve of war with Hoshido.
Honestly, they would probably keep that under wraps. Even more so, if scouts returned with word that his kids were aiding "the enemy".
There's also the moral hazard of putting ones family in jeopardy in pursuit of "doing the right thing".

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Edit;
@GM-sama, What class/options would be open to us?
Or are we using classes and their restrictions more as guidelines than any kind of hard rules to follow?