Man, the last few chapters got me thinking - what if the author pulled out the "Hiyori never actually existed" card? Like if she actually was Mitsuki's other personality, developed as a side effect of neglect/abuse? And if Yuuya's analysis was correct? He would instantly change from stereotypical dense-dumb-male-MC stereotype to at least a very good character.
I think it would make this manga an 8.5 at least.