Mr. Tanabe is the strict homeroom teacher of Class 1-2, overseeing students Takagi, Nishikata, Mina, Yukari, Sanae, Nakai, and Mano. A middle-aged man, he sports short black hair swept slightly upward, very small black pupils, and a neatly trimmed mustache. His standard school attire is a black tracksuit, reserved for formal events like entrance ceremonies.

He enforces classroom discipline rigorously, showing little tolerance for misbehavior or insubordination and frequently punishing disruptive students. Nishikata draws particular attention due to his loud reactions during contests with Takagi, sometimes resulting in chalk being thrown at him. Mr. Tanabe strictly upholds school rules, confiscating holiday chocolates on occasions like Valentine's Day or Christmas. Despite this stern approach, he occasionally shows leniency, such as permitting a student to give chocolate to a friend against regulations. He also reveals unexpected emotional depth, moved to tears during a class play at the school culture festival.

His personal life evolves significantly when he proposes to his colleague Tsukimoto-sensei, Sanae Tsukimoto's older sister. Their engagement leads to marriage, after which Sanae addresses him as "Nii-san". In live-action adaptations, his role expands. He becomes more involved with the main characters, observing the dynamic between Takagi and Nishikata and attempting to boost Nishikata's confidence. During Takagi's potential move away, he offers Nishikata encouragement about his feelings.

A decade later, depicted in the live-action movie, he advances to vice-principal. In this role, he supervises Nishikata and Nakai while welcoming Takagi back to the school as a practice teacher. He remains the only character portrayed by the same actor across both the live-action series and film.