Congrats ahopp, came out great. Regarding the background, the happy feel of it didn't bother me particularly, it feels very cute and lively, fine for a cartoony warrior really. Of course if that's not what you want then it's not appropriate

The only thing that jumped out a little bit to me was the shadow behind him, because the colour is implying that the gradient is a sky representation, but the shadow suggests the gradient is just there for graphic purposes, leaving it somewhere in the middle. So I suppose my suggestion would be to decide on if it's going to be sky, or just an abstract background, and tailor it for that.
About your last question, I remember them saying we're welcome to put in an environment but it will have very little effect on the overall score, which I guess means we're being judged almost solely on the model... almost...