- "Okay, pipe down you hooligans or you'll be in detention till Christmas yet to come!"
- ―Muriel in Yes, Mikey, Santa Does Shave
Miss Muriel P. Finster, often referred to as just Finster, Miss Finster, or Muriel, is an assistant teacher (later fifth grade 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.
She lives in an apartment, as shown in the episode "Weekend at Muriel's," in which she calls it "[a] pad." As seen in the 2003 feature-length film, Recess: Taking the Fifth Grade, she becomes the gang's Fifth Grade teacher. She is voiced by April Winchell.
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 of 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 where 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. Particularly in the 2001 feature-length movie "Recess: School's Out."
- 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"
- Miss Grotke: In Recess: School's Out, when she learn that Grotke is secretly trained in some form of martial arts, this impresses Muriel, who develops a newfound respect for her. Muriel even suggested a friendly match between her boxing and Grotke's martial arts. Though the latter says her skills are only meant for self-defense.
- T.J.: She treats him like a hooligan, but according to the episode Nobody Doesn't Like T.J., she likes him. She came to his rescue when he was nearly attacked by Bald Guy.
- 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. They met again thirty years later, Phillium wanted to reconcile with Muriel but she now sees him as a "recess hating pretty boy" and was very disgusted by his actions. In the end, she helped defeat him along with the students and teachers to protect summer vacation and he was arrested.
- Principal Prickly: He is Miss Finster's best friend, they have known each other for decades and are very close. It is clear they are platonic as he is married and Muriel had interest in other men.
- Randall: Randall is Ms. Finster's assistant and does her dirty work. Despite their friendly status, Muriel has been shown to punish him like other students as shown in Tattletale Heart.
- Miss Finster had a broken hip and phlebitis in 1983.
- In the episode "The Biggest Trouble Ever," it is stated that she is a year younger than Prickly. She is also shown in this episode to have played on the statue of Thaddeus T. Third III.
- Miss 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 portrayed 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.
- Miss Finster entertained soldiers in the Pacific and Guam in 1953 (as stated by Ms. Finster in "Weekend at Muriel"), and 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.
- She was much happier when she was younger this was before Phillum Benedict tried to take away recess.
- She mostly allies to the children as they behave, but she was seen as an enemy to the children when they misbehaving (but neutral).