Hansuke Doi  

, 土井 半助

Rantarou's main teacher is someone who causes him a great deal of stress and anxiety because of his students' inattention and forgetfulness when it comes to theory. despite this, he is highly respected by his pupils and does not tolerate anyone looking down on them. although he has expertise in explosives and strategy, he often resorts to using school supplies such as chalkboards, erasers, or attendance books instead of actual weapons during confrontations. kirimaru accompanies him during holidays, and the two have formed a close bond, almost like family.