Next to Tom Sawyer, Injun Joe is the most important character in the novel. During a boy's maturation, he must sometimes encounter evil in its most drastic form, and it is through Tom's reactions to Injun Joe that we most clearly see Tom's growth from a boy into a young man.
Injun Joe is a thieving, dishonest, wicked person who achieves most of his evil goals because he is also clever and resourceful. He kills young doctor Robinson without qualms and for no discernible reason except for pure evil pleasure. He frames old Muff Potter, and he is shrewd enough to make the townspeople believe his story is true. When proof of his part in the murder is about to be revealed, he reacts quickly and decisively at the trial: He takes immediate action and jumps out the window and escapes and cannot be found by the search parties. In addition, his reputation is such than none of the citizens will confront him with his evil. Although all the citizens of St. Petersburg know that he is evil, each is too frightened to confront him because they, like Tom and Huck, know that he will retaliate in a violent manner.
Injun Joe is a static character, that is, he is the same at the end as he is in the beginning. He does not change through the course of the events in which he is involved. He is the essence of evil when we first see him murdering Dr. Robinson and framing Muff Potter for the crime, and he remains the essence of evil throughout. Consider, for example, his plan to mutilate the Widow Douglas in retaliation for something her late husband did years earlier.
Injun Joe is central to the novel's primary adventure and appears in some of the most important scenes in the novel: He is first seen murdering Dr. Robinson and framing the innocent Muff. He flees justice at Muff Potter's trial. He is the central figure in the search for buried treasure; he shows up, disguised as a deaf and mute Spaniard, in a haunted house where Tom and Huck are hiding upstairs. Later, he displays his extreme cruelty as seen in his plans to revenge himself on the Widow Douglas. When he threatens to kill his partner if the latter refuses to help him mutilate Widow Douglas, he simply reinforces his evilness. Tom encounters Injun Joe in the cave, where he is finally trapped with his ill-gained gold and dies a befitting but horrible death.