Did the mangaka run out of funds and couldn't afford the bathing water?
What would a demon lord to in such a situation? Ravishing her! It's simple. So simple, that he should have known that, too. Pussy. Either decline her, take her on or build a harem. (He insists on demon lord play, so I make an exception (as the story doesn't suffer to badly up til now, which would be the norm for harem stories) and acknowledge his wish not building up a normal, happy family. As long as he doesn't hurt the girls and doesn't insist on being a good person. Also he is young. Have some fun.)
Shera is an average loli character?