Moral complexity in fiction is often confused with moral ambiguity, but they are different things. Moral ambiguity means the reader cannot tell whether a character is good or bad. Moral complexity means the reader can tell, and the answer is: both, genuinely and simultaneously, without the novel resolving the contradiction. The characters worth calling morally grey are the ones whose good intentions produce harm, or whose harmful actions produce good, or who operate under genuine constraints that make easy moral judgment a luxury the novel refuses to provide. They are not redeemed by their virtues or condemned by their failings; they are held in the full weight of both.
The Remains of the Day by Kazuo Ishiguro centers on Stevens, a butler who served a lord who harbored Nazi sympathizers, and who organized that service into the defining achievement of his life. Stevens is not an evil man — he is a man who constructed a framework for moral abdication and called it professional dignity. Ishiguro shows the genuine virtues within Stevens's character — his care, his commitment, his aesthetic investment in his work — alongside the terrible cost of deploying those virtues in service of institutional loyalty. The moral grey in Stevens is not that he couldn't tell right from wrong; it is that he constructed a system for not having to try.
Beloved confronts what may be the hardest moral situation in American fiction: Sethe killed her infant daughter rather than let her be taken back into slavery. The novel asks the reader to hold two things at once — that what Sethe did was an act of profound maternal love, and that what she did was a killing. Morrison does not resolve this by declaring the act justified or unjustified; the novel is about the weight of both truths coexisting. Baby Suggs, the community, Paul D — each character's relationship to Sethe's choice is different, and none of their moral responses is presented as the correct one.
The Brothers Karamazov's Ivan Karamazov is the most fully developed morally grey figure in the Russian tradition. Ivan's intellectual rebellion — his refusal to accept a God who permits the suffering of children — is presented by Dostoevsky with complete philosophical seriousness; the argument Ivan makes in "The Grand Inquisitor" is not refuted within the novel. Yet Ivan's ideas have direct consequences in the world of the novel: his philosophical framework provides Smerdyakov with the justification for murder. Ivan did not commit the murder; he created the conditions under which someone else could commit it without feeling guilty. That is a form of moral responsibility that the novel holds Ivan to without excusing him or condemning him cleanly.
F. Scott Fitzgerald's Jay Gatsby is morally grey in a different register: a man who has achieved the American Dream through criminal means in order to recover a woman who has married someone else, whose fundamental project is a lie, and yet who is depicted by Nick Carraway — and through Nick by the reader — as the most vital person in the novel. Fitzgerald is not endorsing Gatsby; he is showing how a certain kind of American aspiration is constituted by moral corruption, and how that corruption can coexist with genuine romantic energy. Gatsby is a swindler who genuinely believes in something, which is both more forgivable and more damaging than simple cynicism.
What all these characters share is the property of being genuinely better and worse than a simple moral accounting would produce. The novel's achievement with a morally grey character is not to make the reader uncertain about them — that is mere ambiguity — but to make the reader certain about incompatible things simultaneously. Stevens is both admirable and complicit. Sethe is both monstrous and devoted. Ivan is both philosophically right and practically destructive. The moral grey is not a failure of judgment; it is a form of accuracy about what human beings are actually like under pressure.