- "Okay, pipe down you hooligans or you'll be in detention till Christmas yet to come!"
- ―Muriel in Yes, Mikey, Santa Does Shave
Ms. Muriel P. Finster (voiced by April Winchell) is an assistant teacher who is the children's constant nemesis on the playground, and is the main antagonist (and sometimes an anti-hero) of the series. She also serves as the tritagonist of the first movie.
Miss Finster is of German descent. Archetypal authoritarian teacher personality, Miss Finster has a strong desire for a school in which children aren't allowed to have fun or act like children. She is feared by all the children, and is always closely followed by her teacher's pet, the equally nefarious Randall J. Weems, who is also largely disliked by the other students. Her disposition plays an important part in maintaining stability within the society of the playground providing a common enemy for the children of Third Street School, as shown in the times when she is not fulfilling her duties and the playground devolves into a war zone.
Despite her love of harsh punishments, even she has a limit to how far things should go. Such as being visibly shocked when she hears that the Mayor's Council has planned to split the gang send the children to different schools.
Despite her authoritarian nature she has shown to be a deeper and kinder person at times. Outside school she has shown to be friendly, fun and supportive of others as shown with several interactions with the gang outside of school.
During her early years as a teacher Muriel was actually slim and attractive, though she still had the exact same gritty voice. She also had a softer approach to teaching for all those years up until she taught 'The gang' in Kindergarten were she had managed to get them to behave, until Gus Griswald managed to get the class to act like tribal children again.
Despite her age, Finster is an accomplished fighter and is generally in excellent physical condition, demonstrated on a number of occasions.
- Hank: Finster has been shown to have a crush on the school's janitor, but they later broke-up when it was causing problems in their jobs, As stated in "To Finster with Love"
- T.J.: She treats him like a hooligan, but according to the episode Nobody Doesn't Like T.J., she likes him.
- Spinelli: She is friends with Spinelli's Parents and her maternal grandmother. One weekend, Spinelli slept at Finster's, because Spinelli's parents were at a conference. There are even moments where Miss Finster gives advice to Spinelli when she's in trouble.
- Dr. Phillium Benedict: She used to have a relationship with him in the 60s, but broke up with him after he tried to cancel recess.
- Principal Prickly: He is Miss Finster's best friend.
- Randall: Randall is Ms. Finster's assistant and does her dirty work.
- Analogy - Finster is to Randall as Prickly is to Finster
- Ms. Finster had a broken hip and phlebitis in 1983.
- According to The Biggest Trouble Ever, she is a year younger than Prickley.
- Ms. Finster tries to hide her secret friendship between her and the Recess Gang.
- Her surname "Finster" is the German word for "sinister."
- Her surname "Finster" also rhymes with "spinster" which means "an unmarried woman, typically an old woman beyond the usual age for marriage." Many spinsters are often potrayed as very mean, probably due to the fact that they aren't married. This describes Ms. Finster well. She's not married, even though she briefly dated Hank, the janitor. With her gray hair, it may be implied that she's at least sixty, past the usual age when women do get married.
- Her age is very difficult to figure out. In one episode, it was stated she had entertained soldiers in the Pacific, which had to have been in the 1940s or 50s, yet she was displayed to be young during the sixties in Recess: School's Out! In all considerations, that would likely place her in the range of about 60-80 years old.