Agatha Trunchbull is the main antagonist of the novel, film, and musical versions of Matilda. She was the former principal of Crunchem Hall Elementary School, Miss Honey's aunt, and Matilda's arch-nemesis.
Agatha's also afraid of black cats and ghosts. She even hates all children and thought nothing of literally hurling them vast distances and confining those who dared to oppose her into a medieval torture-device that resembled an iron maiden called the Chokey.
Her tortures were so extreme they could easily have been fatal (though by some miracle her victims always survived). She also berates, torments, and orders around a selfless, innocent teacher named Ms. Jennifer Honey who is in fact, her niece by the name Jenny.
She then sent everyone to the assembly room, and called up a boy named Bruce Bogtrotter. She told him that he was a vicious sneak-thief by eating her favorite snack, chocolate cake. As punishment, she then made Bruce consume an entire chocolate cake in front of the whole school. After Bruce devoured the entire cake, she was furious that he had won and she then punished all of the students by making them stay for five hours after school and copy from the dictionary.
In the film, she even put Matilda in the Chokey, thinking she was responsible for her father's lowlife business. Thankfully, Miss Honey rescued Matilda from the dreadful iron maiden.
Agatha attempted to drive to the Olympics, but the car ran out of gas, much to Agatha's fury and anger. When she enters the house, she goes on a rampage, trying to find Matilda and Miss Honey. Luckily Matilda and Miss Honey escaped from the tyrant's house.
Thankfully Agatha was toppled by Matilda's quick thinking and magical powers, who tricked her into believing she was being haunted by the ghost of her brother-in-law (who, it is implied, may have been murdered by Agatha in order to inherit his estate) - upon being convinced of this haunting, she fled in terror.
She arrived to school to teach the children a lesson, about who was responsible for trespassing in her property, and Matilda, now fed up with her torturing, abusing, and cruelty, used her power to use a chalk, creating a message from her deceased brother-in-law, which terrified Agatha. Matilda then sent two chalk erasers to attack her, knocking her to the ground. When she came too, she threw a boy out the window for no apparent reason, but Matilda's magic power sent him flying back at her, causing Agatha to crash on a globe. Matilda then made the globe spin around and around and Agatha went flying off and landed on the floor unconscious. When she came too, she charged like a bull at Lavender, but the latter was saved by Matilda's power, and Agatha crashed right through the door into the hallway. She was then pelted by lunch food by all of the children in her school, as they were all fed up with being tortured and abused. Agatha drove away in her car, and she was never seen or heard from again after this. She's either living in hiding now or was driven to suicide.
In a scene in the book, she holds up a boy by his hair because it was too long. In the film, she holds up a similar boy by his leg instead for not emptying his pockets fast enough. She is portrayed by Pam Ferris in the film adaptation.
In the musical, she is more menacing and cruel over the kids and her niece. At the school she said at the end she make more chokey for locked up all of them, Matilda use her magic power for write on the board as Magnus ghost, Agatha is chased by a bunch of revolting children who told her to run and she run away from the school.