|Cartoon debut||"Arthur's Substitute Teacher Trouble"|
Ms. Tremello was a past substitute teacher for Mr. Ratburn's class. The class disliked her and ranked her as the worst substitute teacher out of every one, because she mumbled everything she said. Like Mr. Ratburn and his sister Miss Ratburn, she is a rat, but has a pale complexion and orange hair, and is likely more than 20 years older than them, but is unrelated to the Ratburn family. Her name "Tremello" is a play on the word "tremolo", meaning "sound effects repeated from a single tone". She looks very similar to Mrs. Featherfoffer.