What is the point of this argument?
As far as I can tell, the point of contention here is that Raineh wants their character to have seen combat in the past, given their military background and to justify their hatred of undead, and SorO feels that if they had been in a fight in the past, they would have xp, and thus not be a starting character.
Is that correct? Because currently, you are both kinda going off on a tangent, and I'd like to start playing sometime this year (given that everyone has their characters done, we are solely waiting on a resolution to this argument.)
I think 5e clearly and unambiguously supports the proposition "Your character has done stuff in the past, even as a starting character". If SorO continues to wish to disagree with this statement in general, then how about we have Raineh's character have been on patrol on the day of the battle, missing the fighting (and thus being all the more impassioned - they won't be absent and unable to save their friends again".
Do either of those solutions work for you guys?