Nigel Raturn (also called Mr. Ratburn and Mayor Ratburn) is a rat who was the teacher of Arthur's third-grade class, until he was instated as mayor of Elwood City after the Inter-species Revolution (Mayor Ratburn). During his time as a teacher, he was notorious for teaching extremely advanced subjects and giving out oodles of homework.
Mr. Ratburn was involved in a scandal in which he threatened Muffy Crosswire, a very wealthy student, and stole money from her. Eventually he went on a shooting spree when he didn't get more money, which resulted in a shot to the head of Muffy (The Ratburn Scandal).
In the Volume 6 issue Ratwater, Part 1, it is revealed that Mr. Ratburn has a secret relationship with another third-grade teacher, Miss Sweetwater. It is unknown how long they had been seeing each other before the issue.