Here's a fun question: Satan, real or fictional? I've included other purported deities on this list, but that's not really the debate I'm inviting here. The Satan I'm talking about is a very specific one, the character Parry from the sixth book in Piers Anthony's strange and masterful Incarnations of Immortality series, For Love of Evil. As I've mentioned before, Anthony is a weird dude, and his sci-fi/fantasy has some serious sexual obsessions that at times verge on misogyny. But he is a talented world-builder with a top-notch imagination and narrative chops, and these three qualities are very much on display in his Incarnations cycle.
In this reality, a near-future where science and magic co-exist, human destiny is managed and controlled by several immortals, each incarnations of primal forces: Death, Time, Fate, War, and Nature are the five major incarnations, and the first five books in the series focus on each in turn, and their struggle against evil. Evil, it turns out, is an Incarnation as well, as is Good represented by the Satan and God of antiquity. The brilliant device in all of this is that each of these Incarnations are temporary offices, held by former mortals who ascend into the roles for a time before yielding them to others. And in Anthony's byzantine, incestuous style, there are threads of family and friendship that connect almost all of the officeholders at the time of his tale, making for an intensely personal conflict at a cosmic level.