Shadoo

A shapeshifter like Mimi. He is seen only in Super Paper Mario. He is located in the 100th room of the Flopside Pit of 100 Trials. In order to face him you had to do the Pit of 100 Trials twice. The second time you enter the room he will confront you as a dark version of luigi, mario, bowser, and Peach. All of these forms have 100 HP. Their all fought one at a time.