Muff Potter is the local outcast. He is a dim-witted drunkard on the fringes of society who is often taken advantage of and scorned by the people of St. Petersburg. While Muff considers Injun Joe a friend, the villain only sees Muff as an easy scapegoat. He is not clever enough to think for himself and goes along with whatever Injun Joe says, even if that means taking the blame for a murder he did not commit and of which he has absolutely no recollection. In the aftermath, readers are compelled to look at Muff with sympathy and pity. He is frantic and devastated that he allegedly killed someone and remarks to Injun Joe, "I wish I may die this minute if I did." After Injun Joe betrays Muff and turns him in, Muff is terrified as he is brought before an angry crowd and even convinces himself that it is for the best that he is hanged. 

The townspeople readily accept Muff Potter's alleged guilt, reflecting the hypocrisy and fallibility of the adult world. Though there is no real proof indicating Potter's culpability, the town is quick to assign blame, prompting Twain to comment that "the public are not slow in the matter of sifting evidence and arriving at a verdict." Twain also explores how public opinion defines personal opinion; after Muff is proven innocent, the town ignores his previous status as the town disgrace and embraces him. Twain reflects on this "fickle, unreasoning world" that now "fondled [Muff] as lavishly as it had abused him before." Muff Potter is therefore somewhat juxtaposed with the rest of adult society. One of his defining traits is his kindness. Before his imprisonment, he befriended the children of St. Petersburg, often mending broken kites and sharing good fishing spots. Even while facing imminent execution, Muff stops himself from dwelling on his sentence so as not to upset Tom or Huck, and urges the boys to stay away from drinking so that they do not end up like him. In a novel that often criticizes the adult world, Muff stands out as an entirely sympathetic and kind-hearted grown-up.

Muff Potter also helps develop the novel's titular character. Tom feels guilty after Muff is imprisoned and brings small gifts to his jail cell. Eventually, Tom's conscience eats away at him and he comes to Muff's defense, testifying that Injun Joe has framed Muff. Tom's decision to sacrifice his own safety and risk Injun Joe's wrath in order to save an innocent soul is the first step he takes on his journey toward maturity.