Onsen-Mark is the homeroom teacher to what has been dubbed as "The Classroom of Failure." The 2-4 homeroom. Among his students are Lum, Shinobu, Megane, Kakugari, Chibi, Perm, Mendo, Ryuunosuke and his greatest nemesis as a teacher, the infamous Ataru Moroboshi.
All in all, Onsen is a born loser. He is a middle-aged man who lives all alone in a shabby boarding house. He puts all of his energy into teaching but has absolutely no control over his students. They just run amok while he desperately tries to keep them in line. But it's an exercise in futility. They throw parties, skip classes, eat and talk during lessons. Then there's those uninvited guests that keep showing up. Almost every week the school ends up being demolished, and he ends up getting the blame. His students find it all too easy to make a fool of him, but he is also quick to retaliate and often uses ruthless tactics to try and control the students. He often puts his class through boring and unenlightening English lessons which are a hallmark of Japanese teaching. There is no bigger loser on the face of the earth than Onsen-Mark.
Additionally Onsen has a crush on Sakura and to a more innocent extent for Lum, neither of whom he has a chance in hell with.
Notes about the name
Onsen are Japanese hot-springs. Onsen-Mark has little hot-spring symbols on his clothing which alludes to the meaning of his name, "Mark of the Hot Spring." His name is tricky to translate. In one respect the "mark" is a suffix on his name. But in another, it's also his given name. It doesn't translate well to the english naming convention of given-name/family-name.
Voice: Michihiro Ikemizu
Onsen-Mark's voice is done by Michihiro Ikemizu who also plays Isao Ohta from Patlabor (surprise, surprise). He was the voice of Gekkou Kamen which is an old yuusha show that I guarantee most of you will not remember.