Isao Ohki is the father of Tsuyoshi Ohki and Aono Ohki (later Tsuyoshi and Aono Sasaki), and ex-husband of Ms. Sasaki. He is both hard-headed and quick to violence, and a direct link can be seen to Tsuyoshi's habit of flying into flings of rage and Isao's own. While he seems cruel and uncaring on the exterior, he is show to care deeply for his son, though he lets his arguments with his mother get between them.
He is arrested a while after he and Ms. Sasaki divorce, and Tsuyoshi is made fun of because of it. He is believed to have attacked random passing teenagers while drunk, but he is later discovered to have stopped a mugging. When he leaves jail, Tsuyoshi picks him up, and bonds more closely with him at his mother's advice.