Doi Hansuke is a twenty-five year old instructor at the Ninjutsu Academy, where he serves as the theory teacher and advisor for the first-year Ha class, the group which includes the protagonist Rantarō and his friends. He is also the advisor for the Explosives Committee. While a capable ninja, his primary role is that of an educator, and he dedicates himself to this position with a great deal of patience and sincerity.

Doi is widely regarded as a kind-hearted and good-natured teacher who is both warm and strict with his students. He genuinely cares for the wellbeing and development of his class, often referring to them affectionately. However, this dedication comes at a personal cost. The poor academic performance, constant rule-breaking, and general mischief of his students are a significant source of stress, frequently giving him headaches and stomachaches. Despite these frequent frustrations, he never abandons his students and firmly believes in their practical abilities and individual talents. Outside of school, he can be somewhat absent-minded, sometimes forgetting to pay his rent or attend community cleaning duties because he is so busy preparing remedial classes for his charges.

His role in the story is primarily that of a mentor and a grounding presence for the young ninja-in-training. He is often seen trying to maintain order in the classroom or rescuing his students from predicaments caused by their own enthusiasm or lack of foresight. In a more practical sense, he is a highly proficient ninja. Although he teaches theory, his own combat skills are formidable, and he is known to easily defeat enemy ninja using nothing but everyday school supplies like chalk, his attendance book, or a blackboard eraser as weapons. He is also an expert in strategy, military tactics, and the use of explosives.

One of Doi's most significant relationships is with his student, Kirimaru Settsuno. Because Kirimaru has no living family, Doi, who himself grew up without parents, acts as his guardian. During school breaks, the two live together in a rented house, where Doi often ends up helping Kirimaru with his many part-time jobs. Their bond is deeply familial, with observers sometimes mistaking them for father and son. His connection to fellow teacher Denzou Yamada is also important; Doi was rescued by the Yamada family during a mission gone wrong years ago, and it was on Denzou's recommendation that he became a teacher at the academy. Despite belonging to different and rival ninja factions, Doi maintains a cooperative and friendly relationship with teacher Kouji Makaino. His most notable rival is a ninja named Sonnamon Moroizumi from the Tasogaredoki clan, though Doi largely considers him a nuisance and defeats him with ease when provoked.

Doi has several notable quirks and a distinct personal history. He has a strong, almost childish, aversion to fish-paste products like chikuwa and kamaboko, a distaste stemming from a past incident where such food became lodged in his throat. His perpetually busy and somewhat disheveled appearance belies his actual looks; he has long, messy brown hair that is almost always hidden under his ninja hood or a casual hat, leaving only his characteristic bangs visible. Before becoming a teacher, Doi was the son of a powerful family who lost his parents and home in a conflict. Following his father's dying wish to forgo revenge, he lived in a temple to study and train, eventually becoming a ninja as a means of survival. This tragic past is the source of his deep empathy for orphaned children like Kirimaru. His personal journey continues to focus on his role as a teacher, finding purpose and a new family in his dedication to his students at the Ninjutsu Academy.