Kotetsu Kakuunsai is a second-year student at Mitsuba Academy and a ninja of the Koga school. He works for the United Kanto Group, an organization connected to the school. In his usual appearance, he has long black hair that is tied back, but he can adopt a very different look by wearing round glasses, which allows him to pose as a timid and bookish student when the situation calls for covert observation. His primary role in the story is that of a skilled agent who provides support to the two main protagonists, Shinokita Reiko and Yajima Junko, acting on instructions from Tokuzen Yukiya. He is exceptionally competent in information gathering and intelligence work, and his Koga ninja training makes him proficient in a wide range of martial arts, allowing him to hold his own in combat. By nature, he is loyal and dependable, but he is also adept at blending into a crowd or playing a harmless role when necessary.

His most important relationship is with Tokuzen Yukiya. The two share a close master-servant bond, but the connection runs deeper than a simple hierarchical arrangement; it is founded on profound mutual trust, with Kotetsu willing to advise or even gently challenge Yukiya when needed. Another significant bond is with his foster brother, Kenbou Kasumi. Kotetsu feels a strong sense of responsibility for Kasumi and is protective of him, often worrying about his safety. Through his missions for Yukiya, Kotetsu repeatedly assists Reiko and Junko, forming a reliable working relationship with them. His personality is marked by a quiet competence and a calm demeanor. He rarely seeks the spotlight, but his contributions are consistently vital. In the course of the story, he does not undergo radical personal change, but his reliability and the depth of his loyalty to those he serves remain constant, and his skills as a ninja and information specialist are frequently put to use in resolving the conflicts that arise.