Doopliss is a major antagonist in Paper Mario: The Thousand-Year Door, debuting as the main boss of Chapter 4. He has the ability to shape-shift and steal a person's body, leaving them as a shadow. He is a member of the Duplighost species. Unlike in the first Paper Mario game, Doopliss is the only Duplighost in the game.
He used his shape-shifting powers to transform the villagers into pigs whenever he rings the bell at the top of Creepy Steeple, striking fear onto the village. Before becoming intertwined with Mario's story, he resided on the top of the clock tower of Creepy Steeple. After his defeat, he takes Mario's body and name, and the player must correctly spell his name to battle him again, but you will only have Vivian as your partner. When the player defeats him and gains the Ruby Star, Doopliss joins the Shadow Sirens as a replacement of Vivian, who joined Mario's party.
He is seen with the Shadow Sirens near the end of the game when Mario is investigating the underground ruins of the Palace of Shadow. During this fight, he has greater attack power and can transform into either Mario or his current partner though without the power of the Ruby Star he cannot steal their body and name. In the epilogue, it is mentioned that Doopliss helped Flurrie when she returned to the stage in the play "Paper Mario".
- During the fight in the Palace of Shadow, Doopliss can transform into Vivian, making the battle similar to that of the Shadow Siren battle at Boggly Woods.
- He also makes an appearance in the Japanese Mario comic, Super Mario Kun.
- The idea of Doopliss taking the form of a darker version of Mario and use's Mario's attacks is very similar to Dark Link, an antagonist from another Nintendo game, The Legend of Zelda.
- Doopliss differs from the previous three Chapter boss and onwards, seeing as:
- He's the only Chapter boss to get fought more than one time in a Chapter (being fought two times in-game).
- He is also the only Chapter boss to be refought in a later Chapter, which makes him both a Chapter Boss and a Regular Boss.
- Doopliss and his Duplighost friends only appeared in Paper Mario, Paper Mario: The Thousand-Year Door, and Super Mario Kun. He has not appeared since then.