"Shut the f*** up! Your teacher will be here in a minute!
— Steinbeck to Cody.
Principal Steinbeck is the cruel and somewhat insane principal of Bowser Junior, Cody, Joseph, and the other students. He loves it when the students get into trouble, so he can punish them. It is unknown why Principal Steinbeck is evil and how he became that way. He is very abusive and mean towards the students, but he appears to be nice and smooth-talking towards other adults, including the teachers.
In "Bowser Junior's First Grade Part 4" Principal Steinbeck arranges a conference meeting with the students and their parents. He says that they "goofed" up after enrolling Mr. Winkle as the new teacher. Bowser gets angry with him and asks why they didn't know he was a pedophile and thinks they didn't do enough backround checks. Principal Steinbeck responds by saying that they tried everything, but has Officer Goodman explain the rest. As Goodman says that Mr. Winkle called him "hot" Principal Steinbeck remarks that he called him "hot" too and started to bicker angrily. After Joseph's Mom reveals they need more "Windex" Joseph remarks that that's not what was going on. Steinbeck laughs evilly and says that the next replacement teacher will be perfect.
In the Part 5 of the 1st Grade series, Bowser Junior answers the door to their apartment and sees Principal Steinbeck. He explains that he needs to see Bowser. So, Junior goes to Bowser's room and tells him that Steinbeck needed to see him. Then, Principal Steinbeck tells Bowser that they'll let Junior pass 1st grade and says that 2nd grade won't be easy.
In First Day of School, Principal Steinbeck barges into the classroom and tells the students that if they in trouble, he'll punish them in the baddest way possible. Cody asks when the teacher would arrive, but Steinbeck tells him to shut up. Afterwards, he's never seen again.
He later returns as a minor antagonist in the Bowser Junior's First Grade series. He replaces Junior's old, original teacher with David Winkle, who later turns out to be a pedophile. He eventually gets Jackie Chu to return as a teacher. When Mr. Winkle returns impersonating Jackie Chu, and Bowser Junior accidentally burns down the school, Steinbeck cuts a deal with Bowser; in order to prevent a lawsuit, Steinbeck promises to pass Bowser Junior as long as he does not sue. He then tells Bowser Junior 2nd Grade won't be as easy, and leaves.
He returns once more in First Day Of School!, when Junior enters Grade 2. His diabolical nature is unchanged, as he tells the students that he hates them and hopes that they will fail and die. He also says that if one of them fails, he will murder all of them.
Likes and Dislikes
Punishing and terrorizing students
He wore a hat in Bowser Junior's 1st Grade! Part 4. He also wore a different suit in First Day Of School!
According to some fans, he is similar to The Joker from Batman.
In his first two appearances, he has a slight British accent, but in later videos, his accent sounds more American.