The naturalist literary movement was an extension of the realist literary movement, a movement that strove to portray life as it really was. However, naturalistic writers saw their writing as a means of scientifically studying human beings, through "objectivity and detachment," without drawing moral conclusions about the human beings (Washington State University, "Naturalism in American Literature"). Emile Zola, a founding father of naturalism, once called human beings "human beasts," meaning creatures that can be scientifically studied by observing them in their environments ("Naturalism"). Developments of the scientific method and social Darwinism both significantly influenced naturalism.
Since naturalism sought to objectively study life as it really was, like realism, which sought to portray life as it really was, naturalistic characters were most often "ill-educated or lower-class characters" ("Naturalism"). The characters also had little control over their lives; instead, their lives were "governed by forces of heredity, instinct, and passion" ("Naturalism"). Any efforts the characters make in changing their lives are thwarted by circumstances out of their control, making them stagnant characters--characters that do not grow and develop throughout the story. Naturalists applied the theory of social Darwinism to develop their characterizations. Social Darwinism teaches survival of the fittest, that only the strongest will survive. Since naturalist characters are low-class characters, they are not born with the ability to change and survive.
While naturalism is often perceived as an extension of realism, naturalism is influenced extensively by the theories of Charles Darwin and his ideas about survival of the fittest, ideas that lead to a certain determinism. A naturalistic character, then, is described as controlled by heredity, instinct, "the brute within" and the environment as well as certain forces of an indifferent and deterministic universe.
One naturalistic character is Tess from Thomas Hardy's Tess of the d'Ubervilles. Tess, an impoverished and uneducated girl, is a victim of forces outside her control. She is sent to work for her wealthy distant relatives and suffers rape and has a child, but the child dies. Tess struggles to survive her misfortunes and seems to have found happiness when she marries Angel. However, after she reveals her victimization by Alec d'Uberville, he rejects her, and she again finds herself destitute after Angel leaves the country. But she procures work on a farm where she must work very hard.
One day Tess decides to walk to the parsonage in Emminster, where Angel's parents live. However, she abandons her plan when she overhears Angel's brothers mention Angel's unwise marriage. On her way again, Tess re-encounters her cousin Alec d'Uberville, who has converted to Methodism. This time, he tells her to stay away from him and she hopes that she is rid of him. But, as fate would have it, Alec turns up in another location and he tells Tess he is no longer a preacher; furthermore, he wants her to come with him this time, insulting Angel when she informs Alec that she is married.
Tess cannot escape misery and poverty. After this new encounter with Alec, her father dies and the lease on the family farm ends with his death. Out of desperation, Tess agrees to become the mistress of Alec (naturalistic theme of sex as a commodity) so that her mother will be taken care of; unfortunately, Angel returns from Brazil and asks her mother where Tess is. When he arrives at Tess's home in a seaside town, Tess tells him it is too late because she has become the mistress of Alec d'Uberville. Overcome with the cruelty of circumstance in her life, Tess blames Alec for causing her to lose Angel twice and the naturalistic "brute within" exerts itself and she stabs him to death. Tess is later captured by authorities and hanged. Certainly, the story of Tess is what naturalists call a "chronicle of despair." Further, all of Tess's misfortunes occur in what seems to be an indifferent universe--Hardy refers to this controlling fate as Immanent Will. Against this Immanent Will, free will seems an illusion.
I'm assuming you mean a "naturalistic character" as in someone with a naturalistic personality.
A naturalistic person is a person who believes in naturalism.
According to a philosopher named Paul Draper, it is
The hypothesis that the natural world is a closed system.
In other words, it is the belief that there are no such supernatural beings that exist at all. It is the opposite belief from supernaturalism, the belief of the supernatural.
Methodological naturalism is a type of naturalism. It is the belief that science and history are the answers to life's problems, which promotes more investigation in a certain topic. For example, instead of saying that a demon was the cause for falling ill, it would be a methodological naturalist's job in order to prove that wrong and find scientific and/or historical evidence for that counter.
Although supernaturalists and naturalists hold opposite views, they are not actively against each other.
In literature, naturalism refers to a philosophical view on the way of writing that tries to impose scientific knowledge on top of it.
As a source suggests,
The Naturalist believed in studying human beings as though they were "products" that are to be studied impartially, without moralizing about their natures.
This emphasizes the science that was trying to be incorporated into this type of literature.
A naturalistic believes that the ways that human beings actually work can be understood and studied (like how the nature of a human being can be studied). They also study how the external environment and stimuli can affect human behavior.
To sum it up, naturalistic literature is just a movement that is an extension of realism literature, where things can be looked to more of as in a scientific way.
The types of characteristics often found in naturalistic writing can be found in the source below, as well as a couple of themes that are used in their writing.
Hope I helped~! Good luck!