The Shape Shifter is a one-time villain in the Phineas and Ferb series, appearing in the episode "Nerds of a Feather". He serves as the main antagonist of a TV pilot for a new television series called "Doof 'n' Puss", based on a story pitch written by Heinz Doofenshmirtz to television producer Jeff McGarland during the Tri-State Area's annual sci-fi and fantasy convention.
He was voiced by John O'Hurley, who also voiced Roger Doofenshmirtz in the series.
According to the "Doof 'n' Puss" pitch written by Doofenshmirtz, the Shape Shifter is plotting to take over Danville with his army of Ninja Vampires. To do so, he captures Danville Mayor Roger Doofenshmirtz and impersonates him in order to secretly write up a law that legalizes Ninja Vampires.
Due to the Shape Shifter's obnoxious and cruel behavior while posing as Roger (such as transforming into a werewolf during a ribbon cutting ceremony), Doofenshmirtz (who is written as a rogue-trillionaire inventor-extreme fighting champion in the story) and Perry the Platypus are assigned to spy on the Mayor. To do so, they disguise themselves as stage magicians in hopes of gaining an act for the Mayor. However, the Shape Shifter learns of the duo's true identities after getting a picture of them buying the disguises from a store, so he sets 20 of his Ninja Vampires to capture them and hold them prisoner inside a storage closet where the real Mayor is being held at.
Perry eventually manages to free himself, Doof and Roger with scissors, and they all manage to expose the Shape Shifter as a fake to the public, forcing the angry Shape Shifter to transform into a giant monster to attack the heroes. It is unknown what happened to the Shape-Shifter afterwards as the story was cut off by McGarland, who decides to pitch the story as "The Platypus and His Girlfriend" when Doofenshmirtz refuses to give Perry a girlfriend in the story.