The unnamed Sensei (Japanese for teacher) is Nobita's teacher. In the English dub he is known as Mr. S. In the Hindi dub, Sensei is simply referred to as Sir.
He is a strict teacher who often punishes Nobita for failing his tests, being late to school, or forgetting his homework. Many of his punishments range from standing in the hallway to sweeping the classroom after-school. The Sensei is often fooled by Suneo Honekawa's convincing acts of lying and is impressed with Hidetoshi Dekisugi's good grades. He often pays unexpected visits to the Nobi’s in order to inform Tamako about Nobita’s progress. The Sensei also usually sees Nobita on the streets and often encourages him to work and study harder.
His real name is unknown and he is only referred to as "Sensei", but in the 1973 anime, his name is given as Ganari (我成).
Sensei is a middle-aged man with a barrel-shaped body and peach skin. His hair is black is in a curtain-style cut, with the sides and back trimmed short. He wears a pair of rectangular glasses, has a fat button nose and notable cheek wrinkles.
Most of his appearances of him are teaching at school and coming back from work at school. He wears a white shirt with black necktie and brown blazers as well as slacks. His height is roughly equivalent to Gian's.
Sensei is a respectable instructor with stern disposition. He is usually strict with his students, especially Nobita and notably Gian. However, he also frequently expresses his care and hope for his students so that they may become better individuals in the future.
Occasionally, when he finds his students (primarily male) by the roadside, he would call them over to give them advice or to remind them to do their homework. Throughout the series, he sometimes visits his students' houses (notably Nobita's) to speak about their performance and test scores with their parents. The Sensei generally speaks to Nobita's mother Tamako Nobi about his performance and progress in school.
- The Sensei is the first unlockable character of Doraemon Gadget Rush.
- The Sensei is one of the adult characters that Gian is genuinely scared of.