Melissa appears as a major character in this T.V. series. She first appeared as an employee of Cookies n' Cream Inc.
Candace and the Busted Squad
She was a hair-shifting villain named Melisdusa in We Call It A Deadly Maze and when she was going home in the Lil Sparks bus, she told them and The Phineas And Ferb Trio that she will get her revenge on Candace and her Busted Squad, so it is true.
She is the main antagonist in this story. Even though she isn't against anyone she is considered an antagonist because she is the one who caused the world to get screwed up. Even though she is the main antagonist in the story she only appears at the beginning of the story and at the end.