He is talked about as an absentee father which is why Huck is in the care of the Widow Douglas and her sister. We don't actually meet him until after the ominous hint of his presence with the impression of the crossed nails in the heel of his boot--indicating that he is a highly superstious character (ranking him right up there with Jim and Huck).
Our first actual encounter with the man is in Huck's room at the Widow Douglas' where he bullies Huck and makes him prove he can read. Mr. Finn is obviously angry at his son for living the dandy life, wearing fancy clothes, going to school, and mostly for having money that Finn wants to get hold of for his own purposes. He left with the only dollar Huck had on him, cussing Huck for trying to be better than his own father. He also told Huck to stop going to school, or he would "lay for me and lick me if I didn't drop that."
This is all in Chapter five, entitled "Pap Starts in on a New Life".