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.