Elmo is a red Koopa Troopa in Monster Mix-Up. He is called the stupidest Koopa Troopa by Bowser (who also states that red Koopa Troopas are already particularly unintelligent). Bowser summons him after he captures the Mushroom King so that Elmo and the king can be combined through his Monster Mixer. However, Elmo does not appear, and Boom-Boom nervously explains that, as he wanted to take a break from guarding the fortress, he left Elmo in charge. Bowser is angry enough to throw Boom-Boom into the Monster Mixer with a Firesnake. Later, as Mario and Toad break into Bowser's Castle, they find Elmo and the king after being mixed together, with the king's head and the Koopa Troopa's body. The resulting individual, named the "king-Troopa" in the narration, is being held prisoner with Princess Toadstool until Mario and Toad find them. The king-Troopa, although being the result of combining two dim characters, is apparently twice as bright now, telling them exactly where Luigi is being held. Mario does not trust the Elmo part of the mix, but refusing to listen to him results in a Game Over from stumbling into a death trap. He also explains how to solve some of the puzzles in the castle, which is necessary to continue. By the end of the story, the group gets to work splitting everyone from their amalgamation, including Elmo and the king.