Pap Finn is a drunk, illiterate, and particularly shallow man. He uses Huck's loyalty in a way that he feels will benefit him, but he also tries very hard to be sure that Huck will not be able to gain any power over him, physically or mentally, even socially. He is particularly interested in Huck as a potential source of income, perhaps as such more than he was ever interested in him as a son or part of a family.
Many would suggest that Twain uses his father also to portray the attitudes of white southerners that had serious problems with freed slaves and African Americans in general. Thankfully after Huck escapes from the shack, Pap dies.
Ugh! If anyone is an example of WHITE TRASH, it is Pap. He is a worthless drunk who beats his son, tries to steal his money, and is prejudiced and illiterate. He kidnaps Huck and subjects him to repeated beatings while he is drunk. He is jealous of Huck's good fortune to have been "adopted" by the Widow Douglas and accuses Huck of thinking that he is better than his "old Pap." He is cruel even when he is not drunk, and does not have any redeeming value whatsoever as a human being. He does not regret beating Huck when he sobers up, either. He is slovenly in appearance, does not bathe, smells, has stringy hair and an evil-look on his face all the time. Huck is petrified of him.