I can see how Nancy was swayed by Steve's gorgeous hair and attractive face. Truly I can. But in the beginning, that's all Steve really had going for him. He wasn't very nice — even going as far as breaking Jonathan's camera when he got too close to Nancy. His idea of fun was shotgunning a beer in his backyard and generally being an assh*ole.
My fear about Steve during the first season was that his character was going to be nothing more than that. He'd just be Nancy's crappy boyfriend who contributed nothing to the show. Thankfully I was wrong. He hit a turning point toward the end of the season, when he helped Jonathan and Nancy fight the Demogorgon, and that really started to paint him in a new light.