I've read a lot of the novel, pretty good series honestly.
To address some comments, yes the MC and the girl on the cover do the dirty dead. He's 15 and she's 14, high school aged kids banging isn't uncommon. I don't really get the drama about it.
Is this series a harem? I guess it is. 250+ chapters in he has a girlfriend (cover girl) and they have a single female companion, and one other non-human companion. Him and the other girl have a few romantic encounters so i suppose it's going to go harem. Not really to me satisfaction. Even if there is... I really wouldn't call 2 girls, or even 3 a harem. I'd say that's more of a polygamy while having 5+ wives with common no development a sure shooter for harem.