Shimizu and Aoyagi looks a bit like the Main Cast from "My Ex-Wife is Young Again and She's in my Class" so my money is that they have some complicated romantic related past.
Not going to lie. That Soul Bond spell is pretty handy with the way you can utilize your harem's feelings.
More harem series need to have something like that.
The mangaka has also released some new(not really) art.
It's the one he used for the Cover of the July edition of the monthly Shounen Ace and a piece He used for a previous chapter(Just now without any marks).
It looks like the manga didnt cover this but, around this time in the Web Novel, it described what would happen if Yona never came to help Margueritte and her family.