You don't need to add a superfluous character for that. All you have to do is not address the romance aspect. The core gimmick of the manga is a visitor's guide to Okinawan culture. Lean into that and explore the other friendships within the group. There's plenty of other options without...
If Rei was actually a good brother, he'd tell all the bitches trying to use his brother to get close to him to fuck off. Being nice to them only encourages them to keep abusing his brother.
Cliches are, by definition, things that are commonly seen and understood as to become dull or boring. Unfamiliar things aren't cliches unless it's because of a cultural disconnect.
EDIT: I think I misunderstood your use of "get it". I think you meant you don't enjoy them while I took this...
There's zero chance a government banishes a soldier who is basically more powerful than their entire army because one of their princesses is a thot. That would compromise the security of the entire nation.