Ms. Muriel Periwinkle Finster is an assistant schoolteacher who is the children's constant nemesis on the playground. She is often an adversary, but serves as the tritagonist of the 2001 film Recess: School's Out, which is based on the 1997-2001 television series Recess, in which she serves as the main antagonist.
Archetypal authoritarian teacher personality, Miss Finster has a strong desire for a school in which children are not allowed to misbehave 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.
As shown in "The Story of Whomps", she also shows a sense of sadism by treating make-up words kids think of as swear words and sends them to Prickly's office for a punishment they rightfully don't deserve.
Despite her authoritarian nature, she has been 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, particularly Spinelli.
Despite her age, Finster is an accomplished fighter and is generally in excellent physical condition, demonstrated on several occasions. Former Navy commodore from 1952 to 1954 (Korean War.) Her surname "Finster" is the German word for "sinister".
In her youth, she had long red hair and wore large blue glasses. In her senior years, her hair is short and gray, and she now wears triangular glasses. Her traditional wear is a yellow dress with a diamond pattern.
In the feature film Recess: School's Out, it is revealed that Ms. Finster has been holding back on the school's ice cream supply and intends to sell it back to the district in exchange for chalk and erasers. When Randall briefs her about the kids' last day of school activities, as well as TJ and his friends' absence, she takes no notice and for that her plans are derailed when TJ and his friends catapult the ice cream down on the students. She is then seen with Miss Grotke and Principle Prickly watching the kids leave when the school day ends.
A few days later, Ms. Finster is boxing a punch bag when she hears the doorbell, thinking it is the pizza delivery, but it turns out to be Randall. He informs her about T.J. and his friends breaking in to the school (in reality they were trying to rescue a captive Principle Prickly) and she takes action to have the kids tried. Once there, Miss. Finster and Randall make two unsuccessful attempts to get into the school since the doors are locked (and electrified): First, she climbs up the rope to the window left by Mikey Blumberg and it snaps under her weight. Then she tries going backwards through the basement window, only to get stuck in the window frame ("Curse these bodacious hips of mine!" she adds). While Randall goes back to her house for some butter to loosen her up, Miss. Finster witnesses the tractor beam displaying it's laser ray at the moon and she gets even more suspicious when she sees the gang (minus a captured TJ) being chased by two ninjas.
In Principal Prickly's flashback of 1968, it has been shown that Muriel had been in a relationship with Phillium Benedict, but it ended when she realized that she could never be in love with a corrupt individual. In the present, after failing to convince two policemen, Miss. Finster rounds up the staff to take back the school with the additional help of the students.
Phillium sees her and is amazed that she is still gorgeous as before. She ignores his flattery by calling him a "two-bit recess hating pretty boy". He pleads with her to take him back but she angrily refuses and fights off his men. She saves T.J. from Kojak by beating the man up and telling the grateful T.J., "she's just doing her job". In the end, she offers Ms. Grotke a boxing vs. karate match, but Ms. Grotke is unsure since she only uses her martial arts for self defense purposes only.
When Miss Finster and the recess gang go to Kauai, she goes to the spa for a relaxing massage. At the end of the episode, Miss. Finster is still in the spa getting a massage without even leaving it to check on the gang. She tells the masseuse that they don’t talk about those “Miscreants” on spa property and enjoy her massage without noticing that the kids are waiting for her at the airport to go back home.
In terms of inter-personal relationships, Miss Finster has been shown to have a crush on the school's janitor, Hank, and was even in a relationship with him at one point. However, Hank and Miss Finster eventually broke up after deciding that they were just better as friends.
In one episode, it's revealed that Miss Finster is actually a long-time family friend of Spinelli's mom's side of the family, as she and Spinelli's maternal grandmother, Agnes, have been friends since at least the 1950s. After that, she becomes a motherly figure to Spinelli, offering her advice when Spinelli's bothered by something. Of particular note is that she used to have a much sunnier disposition, and throughout the series, glimpses of this are seen with occasional references to her experiences in Guam.
At one point in the late sixties, she did have a relationship with Phillium Benedict but broke up with him after he tried to cancel recess. Even when they meet up again, she expresses her disdain towards him, which was warranted after everything he recently did.
Miss Finster seems to become nicer as the series progressed.