At lunch one day, Charlotte gives Vendetta a musical greeting card which outrages her. She tells Grudge to get rid off it, which he does by eating it. Vendetta plans to get revenge on Charlotte by making the "best fiend yet". But when Grudge ruins the fiend mix and the card in his stomach goes off, Vendetta fires him and forces him out into the rain. The next day, she shows up with a new fiend named Rubella, who has replaced Grudge.
Charlotte tries to befriend Rubella, and Vendetta tells her to meet them after school at the pier. Later, Vendetta and Rubella arrive to see Charlotte has just gotten there. Vendetta gives Charlotte some concrete shoes which she accepts happily. Vendetta then tricks Charlotte into walking off the dock with her new shoes, sinking her to the bottom of the sea.
To celebrate Charlotte's "death", Vendetta and Rubella attack the other children in school with fiends. At Vendetta's house, she is happy because of the successes she had with her fiends and considers Rubella a friend. Later at school, Rubella gives Vendetta a teddy bear that produces a happy tune. Vendetta throws it away because of the music. Rubella picks it up again and the music keeps on playing. Vendetta falls to the ground and screams for help. Grudge sees this and eats the teddy bear to stop the sound. Relieved, Vendetta stands up again and yells at Rubella for giving her the teddy bear. She prepares to take Grudge home with him again, but then the greeting card and teddy bear in his stomach start making noise. Angry, Vendetta tells both Rubella and Grudge to stay away from her. Later at school, Grudge and Rubella are seen sitting next to each other, meaning that they could have been friends. However, when the townspeople set sail to leave town, she was not on the boat.
Rubella has super strength, but is not very clever as she always says only one word. When Vendetta introduced her as Rubella, she said "Mary" and when Charlotte asked if she wanted to become friends, she said "friends..." She also ate a glass instead of drinking from it, and tried to eat her desk.
- Rubella was named after the Rubella virus, which causes the Rubella disease and the congenital rubella syndrome.
- She was voiced by Elissa Calfin, a friend of Amy Winfrey's.
- The episode Rubella first appeared in was named after her.
- It is unknown what happened to Rubella. When the townspeople left town because of the fiend invasion, Rubella was not seen on the boat. She was presumably left in town.