Keisuke Tanabe is the central protagonist of the manga series Toritsu Mizusho!, serving as a sociology teacher at a unique public high school in Tokyo's Kabukichō district. He is introduced in the first volume of the manga, and his role as a teacher places him at the heart of the story as an educator navigating a very unconventional learning environment. The narrative follows Tanabe, who is described as being on his last chance professionally, suggesting a background where he may have struggled or faced a difficult situation prior to his employment at this specialized school.

As a sociology teacher, Tanabe's personality is shaped by his position as an authority figure in a school that prepares students for the mizu shōbai, a term referring to the nightlife and entertainment industry. He is portrayed as a dedicated educator tasked with helping a student body that the traditional school system has largely given up on. His motivations stem from a desire to provide these young people with a future and a sense of purpose, working against the societal stigma attached to their chosen career paths. He is joined in this mission by a very quirky staff, and together they strive to support their students.

Tanabe's role is not just that of a teacher but also the primary viewpoint character, especially in the original novel on which the manga is based. In the novel, much of the story is told through flashbacks from Tanabe's perspective during his final day at the school, where he is preparing to quit and take over the family bookstore in Fukuoka. This indicates a significant internal conflict between his dedication to his students and the pull of familial obligations or personal doubts about his career. Throughout the series, his development is marked by his personal relationships. He shares a significant connection with Akane Yoshioka, the director of the school's fuzoku (adult entertainment) course, and their relationship deepens over time. In the final events of the series, Tanabe and Akane have a son and are shown visiting the newly rebuilt school together, indicating they have formed a family.

Key relationships are central to his character arc. His bond with the student Mari Oda is particularly important, as she eventually becomes a teacher at the school and replaces him as the focus character in the sequel series. Other relationships include his interactions with the headmaster Shigeo Yagura and the various course directors, as well as his entanglement with student Aki Suwahara. Aki's brother discovers that Tanabe was the teacher who broke her heart, leading to a dramatic confrontation where the brother burns down the school, kidnaps Akane Yoshioka, and nearly kills Tanabe. This event underscores the personal risks and emotional turmoil that accompany his professional life. In terms of notable abilities, Tanabe does not possess supernatural or combat skills; his primary strength lies in his role as an educator. His ability is his commitment to teaching and guiding students through the complex social and ethical issues surrounding the water trade, effectively functioning as a mentor and a stabilizing force within the chaotic school environment.