So, here're backstories that can be completely compatible to this cast, complete with altered backstories (which you can alter if you wish), and it gives excuses for the excluded characters.

However, I have also included Byleth since they have a voice mod by someone else and thus could still potentially be added right here.

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2457311945&searchtext=byleth

There's a mod that can help you copy-paste all of these here.

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2568354863

WARNING: Spoilers for all routes in Fire Emblem Three Houses.

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Due to the absurd length that cannot fit here, I made it into a Google Doc right here.

https://docs.google.com/document/d/1xyj4KQ7Q-AsTnQlsDB4_P_kRpZUk9DNjqkTgvziROY0/edit?usp=sharing