Pap is a one-dimensional character in the novel. He has no good qualities. He is a bad father and symbolizes the evil of seeing other people as commodities to be used: he is abusive, resentful of other people's good fortune (even his son's), and greedy, and he sees people only in terms of how he can use or manipulate them. He will reappear in an amplified way in the King and the Duke.
Pap is a bad father in that he has no empathy for his son and no interest in his welfare. Pap, lost in alcoholism, can only think about his own needs. He is a person who thinks of life as a zero-sum game: even his son's gain is his loss. In this, he symbolizes the corrosive side of individualism.
Pap is also the foil to Jim, who is a good father to his own children and who becomes the good father Huck never had. Jim strokes Huck's hair, a sign of affection Huck never receives from Pap, who only beats him. Jim, too, has the empathy to think about Huck's needs ahead of his own: for example, he will let Huck sleep through his watch shifts rather than wake him.
Pap symbolizes all that thwarts the human spirit, his own and others: in particular, the tendency to view people as commodities to be used, not human beings, an evil that Twain shows includes, but goes beyond, owning slaves.
Yet Pap also points to the good that can come from evil. His outcast status teaches Huck to value freedom and learn resourcefulness.