Joichi Nikaidou (sometimes spelled Jouichi) is a moustachioed gentleman wearing a hat whom Hamada berates after the credits in episode 11 for parking in the wrong spot.
He comes to ask about his son, and Hama-chan implies that his generous donations to the school might be intended to bribe for favours.
He is actually Johji's father, but because he had him with his mistress instead of his wife, Johji took his mother's surname instead of his father's.
He meets up with his son at the festival, as well as Johji's mom and Kozue, who were also there. He apparently neglected his mistress for 2 months.