It's actually cute that Erin secretly read a book about forbidden love between human and demon (If i remember correctly).
Because it could make their relationship became more romantical than just sexual if possible.
Honestly the Hero are better off to be a femboy or something, his personality may not be the best but he has a quite nice but bratty appearance.
Let me ship the Hero and Mars instead in another multiverse where they ran away from their responsibility and become a lover.