OK, there are like three things about copyright that I don't fully get: 1. What counts as copying? 2. What counts as creative work? 3. What counts as fair use?

OK, there are like three things about copyright that I don't fully get: 1. What counts as copying? 2. What counts as creative work? 3. What counts as fair use? For me all this three are huge gray areas. I honestly can't decide if derivative works from this game's characters could be sold freely despite what the original author says under the current US law. Please don't mix trademark into the question, I don't know if Undertale or its characters are trademarked or not. For the sake of clear discussion about copyright let's assume that they are not.