The novel is the epitome of a Realist work for several reasons. First, Twain relies heavily on dialect, his way of adding "local color" to his plot--this type of writing, where authors employ colloquialisms and the customs of a particular area is Regionalism (a subcategory of Realism).
Secondly, the novel's narrator, Huck, is a traditional Realist character. He is a commoner, not a nobleman (as one sees in Romantic works), and he struggles with believable conflicts while being disillusioned with the Idealistic tendencies of his society to gloss over the ugly injustices of pre-Civil War America.
Finally, most Realist writers champion equality for all. While Huck Finn focuses on justice and freedom for slaves, other Realist writers such as Kate Chopin advocated for women's rights. Regardless of the type of individual who needed independence, Realists writers share the commonality of giving a voice to the downtrodden or overlooked members of society.