Ambiguously Brown: It's not entirely clear what race he is, although his mother is implied to have been black.
When asked jokingly by a fan, Gaiman has agreed that Dwayne Johnson/The Rock, who has black and Samoan heritage, would be fitting to play him, stating they are both "underestimated".
Badass: while he doesn't like to fight, he's a very big man and knows how to fight smart.
Beware the Nice Ones: Shadow's a nice guy, but he's also a very large man who knows how to handle himself. Even outside of his physical prowess, Shadow dismantles Wednesday and Loki's plans with a single awesome speech.
Unfazed Everyman: Subverted. He takes it rather calmly that there are tons of deities all all around him. But in a few private moments, he shows that he's almost going out of his mind with all the weirdness, and the drama of trying to get fiercely individualistic gods to work together. Makes sense since he is one himself, so he isn't a pure example of the trope.
Grumpy Old Man: he's often grumpy or pretending to be cheerful and reveals himself to be deeply bitter about the fact that as a god who went out of style in a land that's bad for gods, he essentially has to live off scraps.
Pet the Dog: not often, but he does occasionally get more sympathetic moments, such as when he seems to sincerely regret that he can't truly bring Laura back to life and that she had to die in the first place. He also tells Shadow, his son, that he's proud of him.
Technically a Smile: Shadow notes early on that his smile has a sinister motive just behind it, more bearing teeth than an actual display of emotion.
Thanatos Gambit: Dying was the motivation all the other Old Gods needed to band together against the new. He didn't mind because, once the battle was dedicated to him, he would get more power than he ever had before.
Cool Old Guy: Unlike veteran conman Wednesday and bitter loner Czernobog, Nancy's just fine with passing the days doing his own thing.
Dirty Old Man: He talks about his love of "women with big high titties" in his first appearance.
Sesquipedalian Loquaciousness: quite clearly loves words, something that goes with his mythical connections as a scribe, and telling the stories of people - though how true they are is in doubt, with Mr. Ibis suggesting that he does it for the love of the story.
Laser-Guided Amnesia: Shadow forgets everything about him - including his name - immediately after learning them.
Man of Wealth and Taste: An impression of wealth is all Shadow can remember of him. He wears a suit and drinks expensive whiskey with Wednesday. He also appears to be a god of wealth or similar, able to sense the exchanges of money going on around him.
The Nondescript: His only description is that he's dark-haired, clean-shaven, and his face is forgettable in every sense of the word.
Affably Evil: He's friendly to everyone, tells jokes all the time, and goes out of his way to help Shadow on multiple occasions, even saving Shadow's life when it would have been in his best interests not to. But at the same time, he literally murders children for a living.
Serial Killer: He's been sacrificing a child to himself every year.
Suicide by Cop: When finally confronted with his crimes, he provokes the police chief into shooting him.
Samantha Black Crow
Agent Mulder: Delivers an amazing and lengthy monologue to Shadow about all the things she believes in to get him to open up to her.
Badass Normal: Faces Mr. Town and Mr. Road, and not only does she refuse to tell them anything about Shadow, she makes fun of their names. She also hitchhikes with Shadow, who's kind of scary before you get to know him.
After drinking the water of time and a highly unpleasant process of getting rid of all the formaldehyde, maggots etc. inside her, she's back to her living self, though Loki notes that this is temporary.
Lady Macbeth: Seems Shadow's involvement in the robbery that got him in prison was her idea.
The Lost Lenore: Shadow misses her throughout the novel, and her resurrection does little to help that.
Manic Pixie Dream Girl: Deconstructed. Because of that personality, she cheated on Shadow with her best friend's husband while he was in the clink. She also puts on the MPDG act to charm Mr. Town before gruesomely killing him.