Isaac is the leader of the Devil Busters. He is first found as the boy possessed by a Ghoul in the Lost Caves. During his exploration, Yuuji Kino will find him blocking the exit. After being defeated, he returns to normal but is severely hurt and Yuri Takano will ask if she can take him home to care his wounds. If Yuuji accepts, he will recover and will give 10,000 Macca as thanks for saving his life. Later, he invades Yuri's house and tries to drain her magical powers, if Yuuji visits Yuri's home before a New Moon, Issac will be fought as a boss along Raphael, Gabriel and Michael. If not, Yuri will die and the game will end.