Nobita's teacher, consistently addressed as "Sensei" across official media, remains largely unnamed though some adaptations assign temporary names like "Ganari" or "Eiichirou Senjou". He possesses a barrel-shaped physique, peach skin, a curtain-style black haircut trimmed short at the sides and back, rectangular glasses, a button nose, and prominent cheek wrinkles. His standard attire includes a white shirt, black necktie, brown blazer, and slacks, with a height comparable to Gian.

As Nobita's homeroom teacher, he frequently scolds Nobita for academic failures, chronic tardiness, incomplete homework, sleeping in class, and poor overall standing. His disciplinary actions range from hallway standing to after-school tasks like cleaning classroom windows. Beyond punishment, he sometimes offers encouragement, urging Nobita to study harder and avoid zero marks. He makes unexpected visits to the Nobi household to inform Nobita's mother, Tamako, about exam results, progress, and class ranking. Encountering students, especially boys, in public often prompts reminders about homework.

Professionally, he maintains a strict, respectable, and righteous demeanor, upholding high standards for conduct and academics. While expressing admiration for high-achievers like Hidetoshi Dekisugi, he also reprimands Gian for misbehavior. Suneo Honekawa successfully deceives him with lies and boasts on multiple occasions. Nobita's perception of him shifts between fear, respect, and admiration depending on circumstances. A parallel-world storyline hints he may be unmarried, showing his counterpart dedicated solely to teaching. Gian is explicitly noted as one of the few adult figures he genuinely fears.

Localized adaptations give him titles such as "Mr. S" in English, "Sir" in Hindi, and "Signor Maestro" in Italian. His hairstyle resembles other adult characters in the franchise, including Shizuka's father and Yukari's father from *Obake no Q-taro*.