Jim's logic, compassion, intelligence, and above all, his loyalty toward Huck, Tom, and his own family, establish him as a heroic figure.
But can he help it? We think not. Or is Twain actually just trying to show that a black man can be a complex character? Lewis had corralled the horse and forever earned the respect of Twain, who also praised Lewis' work ethic and attitude. So, to call someone an "uncle tom" is an insult.
Huck may think he's silly not to know that some people speak languages other than English, but, come on, he has a point: why do people speak so many different languages?
Sure, maybe he's a little goofier and more committed to these superstitions than Huck or Tom. Jim's love for Huck, however, extends past their friendship to the relationship of parent and child. In the end, Jim gets what he wants: freedom.
Moreover, Jim has one of the few healthy, functioning families in the novel.
He's a slave.