What makes you think Grimm isn't human??
Witches have familiars, wizards have demons, magic spells have their own power. Humans don't have magic powers. Nick has a magic power, ergo Nick is not exactly human. Yet he hasn't quite noticed (at least not while I was watching.) It would be the first thing on my mind if I discovered my self image was so incomplete and misleading. This obliviousness is why Nick seems so dull to most people maybe. Heroes shouldn't be oblivious to the obvious, stupidity is not attractive, and this show is not about Nick's self-discovery as he grows up. Certainly the cluelessness is why I personally can't invest in Nick.