Mr. Rotter is an undead and presumably from Romania, who teaches Dead Languages at Monster High. He was introduced in the webisode "Idol Threat". In his first appearance, he was voiced by Yuri Lowenthal. Currently, he is voiced by Cam Clarke.
Mr. Rotter believes in order and has little patience for students that try to cause chaos. He is, however, not a strict teacher and enjoys to spark enthusiasm for both his classes and the other ones in the students. This means that sometimes Rotter fails to understand when students are not enthusiastic about some of the classes. He is known for never giving 'A's.
Mr. Rotter is an unspecified type of undead, which coincides with his occupation of teaching Dead Languages as he was presumably alive when they were. He appears to be speaking in a Romanian accent, playing on the Bram Stoker-inspired idea of Romania as the nation where the night is ruled by the undead. His physical appearance closely matches Lon Chaney's portrayal of "The Phantom of the Opera" in the 1925 silent film.
In "Idol Threat", Cleo de Nile had studied all night for a particular test and was offended when she didn't score higher than a 'B+', despite doing the work to perfection. In order to get her rightful 'A' from Mr. Rotter, she used an idol with persuading powers on him and got it.
During the events of "Fear the Book", Rotter was manipulated and tracked by Toralei Stripe to various location where she arranged for the Fear Squad to do something suspicious. As such, Rotter saw Cleo de Nile among a stash of balloons, noticed Clawdeen Wolf had rotten gargoyle eggs in her bag, and overheard Frankie Stein and Draculaura talking about a recipe for stinkbombs. Naturally, he believed they were preparing a prank, something he could not let go unpunished; exactly as Toralei had planned. However, the Fear Squad was aware of Toralei's hand in the situation and in "Desperate Hours" prevented the stinkbomb to be dropped during the graduation ceremony that Mr. Rotter presided. To get back at the werecats, Ghoulia Yelps gave them extra math credits so Rotter would send them off to math camp the entire summer.
In "Ghostly Gossip", Rotter was looking over his students' test results.
After Dead Languages, Frankie Stein and Draculaura talked about how all the languages Rotter taught sounded alike, followed by a mocking imitation of their teacher by Draculaura. Mr. Rotter noticed, but thought she was trying to follow his teachings and so gave her the advice to let her 'R's roll.
Rotter functioned as referee during the casketball game between Monster High and the West Valley Mages in "Super Fan".
Mr. Rotter helped organize the class trip to Monte Scarlo in "Uncommon Cold". Just before departure, he heard Frankie Stein sneeze and told her that she wasn't allowed to come along if she was sick.