Heijiuzumasa Ōkawa is the founder and headmaster of the Ninjutsu Academy, the central setting of his story. He is a man of advanced age, having stopped counting after turning seventy, and is characterized by a sharp contrast between his legendary past and his unpredictable present. In his youth, he was considered a true genius and a great ninja, possessing immense skill and knowledge. However, in his later years, while still a formidable fighter, he has become notably eccentric and senile, a combination that often creates more problems than it solves for the students and teachers alike.

At the core of his personality is a deep-seated self-indulgence and a fondness for sudden, random ideas. He frequently initiates large-scale, unannounced events such as competitions or orienteering exercises, not for any clear educational purpose, but because he finds them fun. These whimsical projects are a major source of chaos within the academy, and the faculty has supposedly even discussed an agenda item to ban such "sudden events". To add a dramatic flair to his entrances, he has a habit of throwing smoke bombs before entering a room and prefers to exit high places by jumping directly out of windows. He enjoys telling grandiose stories of his past ninja exploits, though the accuracy of these tales is often questionable.

His motivations appear to be rooted in a desire to observe and add excitement to the daily life of the academy, even if that excitement is at the expense of others. He often sends the main student characters on errands, typically to buy him sweets or deliver letters, which he hoards and keeps hidden. While his behavior is frequently troublesome, on rare occasions his unusual ideas can serve a genuine purpose, such as creating opportunities for rival students to learn cooperation. He is also capable of more serious lessons, having once held a cross-dressing class to teach students about the importance of remaining vigilant and not being deceived by appearances.

As headmaster, his role is that of a chaotic overseer rather than a diligent administrator. He serves as an advisor to the class presidents and his position involves little routine management, as he is more focused on his own entertainment. He maintains a key rivalry with Kojirou Taketaka Kino, the lord of Dokutake Castle, with their conflict often channeled through their respective ninja teams. In spite of his difficult personality, he has several close relationships. He is a good friend of Hemuhemu, a ninja dog. He also has two elderly girlfriends, Kaede and Kisaragi, both talented kunoichi who use their disguises to appear much younger; the three of them are on good terms and are often seen together. His granddaughter, Shige, is a student in the kunoichi class.

Over his long life, he has accumulated many enemies, and ninjas from rival factions occasionally make attempts on his life. However, his skills remain sharp enough that he has never suffered from any serious threat, always staying one step ahead. Despite his reputation for being frivolous, he has shown moments of personal development and genuine remorse. In a significant incident, he harshly told the protagonist that he was disqualified from being a ninja, which caused the boy to leave the academy in shock. While his intent was a form of shock therapy, he realized the severity of his mistake, felt immense guilt, and sincerely apologized upon the student's return. His key abilities include the physical prowess and strategic cunning befitting a legendary ninja, allowing him to remain active and combat-ready well into his eighties.