You're favorite mamono is real and is madly in love with you. (No monster energy spill out onto other people.) -BUT- If you already have a girlfriend, then she disappears being replaced by the mamono (No, the mamono is not your girlfriend monsterized). If you do not already have a girlfriend, then someone of high importance in your life disappears instead.
(I know, it's not that long, but I figured it would be better to put it here because it doesn't refer to any singular monster girl)