Doopliss

Doopliss is a character from the game Paper Mario: The Thousand-Year Door. He has the ability to transform into about anything, like a shapeshifter.

He is met first at chapter 4 of said game, where he is called ? and serves as the endboss. After beating him though, he takes Mario's identity and everybody belives he is Mario. The player then has to find out Doopliss' real name. Afterwards he is foughgt once again at the Creepy Steeple, this time accompanied by Mario's Partners. He lives in Creepy Steeple of Twilight Town.

After he is being fought off, he joins the Shadow Sirens as a replacement for Vivian. They all fight together against Mario at the Palace of Shadow (Chapter 8) near the end of the game.

Trivia

 * Doopliss is based off the Duplighost enemies in the first Paper Mario.
 * In most other languages, his name is Rumpel, the abbriviation for Rumpelstiltskin (from the fairytale), who shares the same task with him (the protagonist has to guess his name).