Cletus' Father (only referred to as Pa) is a character mentioned by Cletus.
In all three main appearances, Cletus has two quotes referring "Pa". When he is passed, he may say "Huoooh, I guess Pa was right about me...", and when passing an enemy, he may say "Pa was wrong, and I ain't no loser!".
Who is he? Edit
The quotes do happen to give a few hints about him, they could put it that he was a mentor to Cletus, who said he couldn't race. It can be seen very similar to him being Doc and Cletus being Lightning. Cletus' shown elderliness and that Cletus said "was" in both quotes suggest he is deceased, just like how Doc was killed off after Cars: Race-O-Rama.