Superstition is tied most closely to the character of Jim, though Huck also participates in discussions on this topic. The novel begins and ends with this theme, with Jim significantly attributing his freedom and wealth to a superstition he espoused early in the story.
The first episode of the novel, in fact, deals with Jim's relationship to superstition.
Jim was most ruined for a servant, because he got stuck up on account of having seen the devil and been rode by witches. (The Adventures of Huckleberry Finn)
Jim is a self-described expert on superstition and his stature in this regard is telling. For much of the novel, Jim is depicted as a fool (and Huck is also, to some extent). In his one area of expertise, Jim proves himself to be a person of faith but not of intellect.
He does not rely, however, on his intellect. Jim instead relies on his faith and his faith rewards him in the end.
Jim tells Huck it is his hairy breast that has made him rich again just as he had predicted on Jackson’s Island.
Huck, for his part, is ambivalent about superstition. He does not clearly believe or disbelieve. This ambivalence is very similar to Huck's relationship to conventional morality. Huck is not convinced that Jim should be subjected to slavery, but he is also not sure that society is wrong in this arrangement.
This moral conflict Huck faces in the novel is symbolized by Huck's relation to superstition. Jim is not similarly conflicted internally. His conflicts are purely outward. He is a "calm soul" as belief, conscience and the like are concerned. This stance is rewarded, ultimately, and Jim is freed.
As the novel ends, however, we have seen many instances where that faith could have led Jim to disaster or even death.