Mr. E. was a substitute teacher filling in for Miss Grotke in the episode "The Substitute." This was because she was sick for a week. His full name is never revealed although it is a rather obvious play on the word "mystery." He has zero fear, he is strict and his teaching style is somewhat harsh. He later is revealed to have a soft side, because T.J.'s project "touched him." Apparently, he has had experience teaching in Southeast Asia, Yugoslavia, and The Bronx (New York City) prior to his time at Third Street. Mr. E.'s teaching style is a radical change of pace for the class; however, the class comes to appreciate him in their own way. He, along with many other prior characters that have been influenced by the gang, make cameos in the episode in the episode "Lawson and His Crew." He is a tall, handsome man who wears business type suit consisting of a black suit with a green vest underneath and a white button shirt with a black tie. The Ashleys at one point refer to him as dreamy, implying he is considered attractive especially since they are very particular about fashion and looks. A strict disciplinarian, he demands obedience and dislikes when people talk back to him. His punishments can be somewhat severe, but deep down he is actually a good person. He is able to see the best aspects of his students and bring them out through his radical teaching, which differs from the standard curriculum. It is also shown that he does care for his students, as he helps Gus stand up to a bully and kindly gives advice to help each of them with their project, earning each of his students' (including at the very end even T.J.'s) respect. He respects loyalty, evident when he applauds T.J. for his project which is a get well card for his teacher Miss Grotke. Considering the implications he was involved with the U.S. military, it explains his strict personality.
Friends: Miss Finster, Principal Prickly, Miss Grotke, Miss Lemon, TJ Detweiler, Vince LaSSao, Ashley Spinelli, Gretchen Grundler, Mikey Blumberg, Gus Griswald, Randall Weems and Menlo
Enemies: Earwin Lawson, Gelman, Conrad Mundy, Greg Skeens, Sue Bob Murphy, Lazy Kid, Chucko Kowalski, Jocko, Buster, Cheay, Koreo and Kurst the Worst
Likes: Helping students who are in trouble, being a substitute teacher and demanding obedience
Dislikes: Troublemakers, misbehaving children, students getting him arrested and being talked back to by any of his students
He also makes a cameo appearance in the first part of "Lawson and His Crew;" applauding after Randall and Kurst's dance recital. This is a reference to Mikey and Spinelli's recital in the episode "Dance Lessons."
Mr. E. may well be a play on the word "mystery," a typical ploy in English literature.