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 Moroboshi Ataru'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 Mendou Shuutarou and other honor students, which he uses to spy on students who break school rules.
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.
In the beginning of the manga, such as in Chapter 15 when Ataru is abducted by a kappa, he is depicted as a strict teacher while still showing genuine concern for his students. As the series goes on, however, he becomes more antagonistic towards his students, particularly Ataru and his friends. He is also often assaulted by Ataru, Shirai Kousuke, or Mendou 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.