|First appearance||Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey (2018)|
- “I am here to play games with your lives.”
- —Dieter, Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey
Dieter is the leader of the Best Fitness Friends and a major antagonist in Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. He is a tall figure wearing yellow and black clothing with a prominent tie, and takes charge of the group's operations. He uses various German terms, including telling Bowser Jr. "auf wiedersehen" (farewell) before trying to kill him. This is likely due to "Dieter" being a German name, while in English, it fits in with the food-based theme naming the other Best Fitness Friends have (in his case "diet", as in forsaking meals for the sake of losing weight). He is a ranged enemy in battle, throwing cards, and can use the Doppelganger Captain Command.
After this, he is next fought alongside both Kaley and Beef in the final battle, where they all take Malatone Formula:X, and after their defeat, merges with them to form the final boss before being defeated for good.
- Bowser Jr.'s Journey profile: The brains of the BFF. He will achieve his goals by any means necessary.
- The colors of the three cards he holds in each hand correspond to each member of his team (yellow for himself, red for Beef and blue for Kaley).
Names in other languages
|Slim; referred to as「スーさん」(Sū-san) by Kaley|
|Italian||Smilzide||"Slim" with the suffix "-id"|
|Spanish (NOE)||Alfie||From "alfiler" (pin). Colloquially, "estar hecho un alfiler" (to become a pin) means "to be extremely thin".|