Do you replace them if you deleted them or do they get replaced by some other miis?
I don’t think you can delete a Mii from your party
@Uniqorned
Only the hero Mii cannot be deleted.
As for the OP's question, I have yet to test it myself, since I'm still using my main party to work on 100% completion and autosave prevents undoing any changes in the Villa when exited out.
Update: i tried and they get replaced by these weird NoName miis
What do you think?