Mad Maddy is one of the granddaughters of Madam Mim. She and Mal were best friends in their childhoods but were drawn apart as they grew up.
Maddy runs into Mal when she returns to Maleficent's ruined apartment. She tells Mal about Maleficent's apartment being ransacked and how everyone had stolen or destroyed Maleficent's belongings. They leave the apartment and go to the Slop Shop to get some 'Slop-Shop Specials' and run into Carlos and Evie. Immediately Maddy and Evie are unknown to one another and Evie takes a disliking to the witch. She refuses to tell them anything about the Anti-Hero meeting and leaves them.
She is next seen by Mal, Evie, Jay and Carlos going into the Evil Queen's cellar to attend the Anti-Hero meeting along with several other villain kids. When the gang sneak into the cellar behind them, she surprises them with cake.
She leaves the meeting early and lures Mal (and unknowingly Evie) away from the Anti-Heros. She leads her down to the Rickety Bridge where her allied VKs are waiting in hiding. She tells Mal that she received a letter from an anonymous source saying that the missing Evil Queen, Cruella De Vil and Jafar would return to Doom Cove after midnight from the Catacombs beneath the island.
However, it is revealed to be a lie. Maddy lured Mal to be attacked by her gang of friends who believe the Anti-Hero club is a futile attempt. They accept their lot in life and refuse to believe they can be good. Maddy was a part of the Anti-Hero club as a spy for her comrades. She tells Mal that she hated her all along and that she was always planning to destroy her ever since she came back to the island. With help from Ginny Gothel, she pushes Mal into the waters surrounding the Isle and leaves her there to be eaten by the crocodiles. But when Evie, Jay and Carlos come to save Mal she flees from the bridge and tells them they will burn like Camelot.
- She and Mal were former best-friends and were considered 'twins' because their hair colour matched. However, after Mal poured lye into her hair and destroyed her dolls she began to grow to hate the girl.
- She probably possesses her grandmother's magical abilities as she was transferred to a witches school on the Isle.
- During her childhood, Mal poured lye in her purple hair (forcing her to dye it), cut the heads off of her dolls and loathed it when she and Maddy her called twins.