Sengoku is one of Hayama and Tsuyoshi's teachers at Jinbou Middle School. He is one of the primary antagonists of the series, continually causing difficulties for Hayama and Sana.

While he gets along with most students and other teachers fine, he has a peculiar hatred of Hayama. When they first meet, Hayama is late to his class, and the punishments he gives out are reasonable due to Hayama's rebellious nature. However, as time progresses, it becomes obvious this teacher only focuses on Hayama, and follows him to the point of stalking, watching for him to slip up so he can have him expelled. Principal Narunaru and the other teachers take notice of his obsession, and usually try to reason with him. However, none can stop him from his burning hatred of Hayama.