Male parent of Maxwell Smart.
Max only ever speaks of his father in the past tense, implying that he is deceased. This seems to be confirmed when 99 tells the Chief that Max has "only his mother", although one would assume the Chief would have already known this from the files [Episode #91: "Diamonds Are a Spy's Best Friend"].
Max's relationship with his father was apparently a distant one, with Max recalling that his father wouldn't even talk to him, and the only thing he ever got him was a pair of long pants so he could go to work [Episode #77: "The Little Black Book, Part 2"].
This may help explain the increasingly father-and-son-like relationship Max develops with the Chief, even calling him "Dad" on one occasion - much to the Chief's embarrassment [Episode #36: "Casablanca"].