Onsen-Mark (温泉マーク or ♨マーク, Onsen-Mark?) is the homeroom teacher of Tomobiki High School's Class 2-4. His real name is unknown, as even the Principal refers to him as Onsen-sensei (温泉先生, Onsen-sensei?). Onsen Mark could even conceivably be his real name, if he is a nikkei (diaspora Japanese), not implausable given that he is an English teacher. In that case his name with English ordering would be Mark Onsen.
Onsen-Mark is Ataru Moroboshi's homeroom teacher and a graduate of Tomobiki High. Throughout the entire run of the series, he is shown teaching various subjects, mainly English, and he is also in charge of student discipline. He formed the Shadow Disciplinary Council, comprised of Shūtarō Mendō and other honor students, which he uses to spy on students who break school rules.
As the series goes on, he becomes more antagonistic towards his students, particularly Ataru and his friends. He is often assaulted by Ataru, Kōsuke Shirai, or Mendō or caught up in explosions and other such unfortunate events that occur in his classroom.
In the manga, he is established as having a wife and child and an elderly mother, but in the anime he is single and lives alone. Because of this, there are many anime scenes where it is implied he is in love with Sakura. In Beautiful Dreamer his dream is shown to be living with a wife who resembles Sakura.
He is a very dedicated teacher, despite dealing with students like Ataru. He will often insist on continuing classes even when the school has been damaged or the class is in an unreasonable state. In Boy Meets Girl, he tries to conduct class outside of the school before being thrown into the sky by a sprouting mushroom.
He is always seen wearing a suit with the onsen mark (♨) drawn on it. In the manga, the symbol is all over his suit and slacks, but in the anime it is only on his tie and on the back of his jacket.
He is depicted as a strict teacher while still showing genuine concern for his students.
- While he was Class 2-4's teacher from the start in the manga, various different teachers were used before being introduced in the anime in Episode 36.
To be added