We would say the snake in the Valley of the Rulers, but he only shows up in one canto, so doesn’t really count as an antagonist. We could be really deep and say Dante…because his sinful self is really what he’s battling against here. But, this is not a psychological novel with profound inner struggles. So we’ll go ahead and say that there is no antagonist. That is kind of the point, though. Inferno shows us that Satan, being frozen and trapped in the center of the earth, doesn’t have a prayer of winning against God. The supremacy of God is a big theme in the Divine Comedy and each of the penitents struggles to reach that glory.