One of the main characteristics of naturalistic characters in literature is that they are mixtures of good and bad qualities, just like real people. John Steinbeck's Of Mice and Men offers many examples. George is commendable for taking care of Lennie, but he sometimes abuses Lennie, and it is questionable whether he did a good or bad deed by killing him. Lennie is good-natured, but he kills little animals and kills a young girl in the barn. Curley's wife is pretty and feels lonely and unhappy. We can sympathize with her impossible dream of being a movie star. But she is terribly sadistic when she threatens to get poor Crooks lynched. Crooks is a lonely, pathetic figure because of being ostracized by the white workers and because of his wretched physical condition; but he shows a mean streak when he torments Lennie by suggesting that George might have abandoned him.
Crooks' face lightened with pleasure in his torture. "Nobody can't tell what a guy'll do," he observed calmly.
Curley is not a likable person, but the reader can feel some pity for him because of his inferiority complex, because of his permanently ruined hand, and even because of his unhappy marriage. He must have thought he loved the girl he married. Candy is getting old and has lost one hand. We feel sorry for him when he is forced to let Carlson shoot the old dog he loves. Yet he shows a cruel streak when he addresses Curley's adolescent wife as she is lying dead in the barn.
"You God damn tramp....You done it, di'n't you? I s'pose you're glad. Ever'body knowed you'd mess things up. You wasn't no good. You ain't no good now, you tart."
It is now commonplace for writers to create characters by giving them one dominant trait and one contrasting trait. This is often sufficient to make a character seem real.
Naturalism is an outgrowth or branch of realism. Literary realism was prominent during the mid 1800's. It is fitting to describe Naturalism as an evolution of realism. The reason that I say that is because Naturalism authors were heavily influenced by Charles Darwin's theory of evolution.
Darwinian evolution, in an incredibly small nut shell, says that an organism's genetics determine how well it is adapted to its environment. That level of adaptation determines the chances of survival.
Naturalism authors applied that same kind of scientific thought to realism. Realism sought to describe how something or someone IS. Naturalism seeks to determine why that person is the way that they are. Naturalism seeks to discover and describe what forces (nature, society, etc.) influence the actions of people. It's quite Darwinian, because naturalism seeks to interpret the combination of Nature and Nurture in an individual.
What I find interesting about naturalistic authors is how they put those scientific links to paper. Nothing that I wrote above focuses on positives or negatives, because science tries to stay objective and neutral. Naturalistic authors claimed a similar scientific neutrality, but I just don't see it. The reason I don't see it is because naturalistic writing tends to be pessimistic and depressing.
It tends to have characters that are lower class and ill-educated. Settings tend toward urban environments (but not always; "Of Mice and Men" for example). Sinclair's "The Jungle" is a good example of that urban environment. Nature is portrayed as completely indifferent, and it will often kill characters that have been a central focus (Crane's "Open Boat"). There's often a feeling of fatalism too. It's as if a character's struggle doesn't matter, because his or her fate is predetermined by their genetics and social situation.
The list of authors often associated with this period and genre did not write happy, cheery books and stories. They are fatalistic, overly deterministic, and depressing. They do not have a glowing outlook on the human species. Crane, Fitzgerald, Faulkner, Hemingway, Steinbeck, Bierce, and Sinclair are all authors associated with naturalism. I haven't read everything those authors wrote, but what I have read was not uplifting to my spirits.