What kind of character is Hally´s father? Why did Athol decided to include this character?

Hally's father is mean and tyrannical. I believe Athol added the character in order that the reader understand exactly what Sam's presence in Hally's life really meant. Sam was not only a surrogate father.... he was the kind of man who could teach Hally about life. To me, the most important lesson for Hally was that there are good men in the world.