I don't see this becoming a harem as of chapter 13. I've always liked H-mangaka (this guy, Cuvie and Akira Goto etc.) doing a proper story because obviously art is always going to be pleasing to the eye.
With that said, I originally thought this is going to end in like 2 volumes, but I'm surprised that there are still quite a bit to go still.
On a random note, I swear Gangan Joker has the most of my favorite series out there (this, Arachnid, Hatsukiai, Love x Rob x Stockholm, Shinigami-sama ni Saigo no Onegai o, Zettai Joousei, Manabiya, Wa! >___<).