Dieter

"I am here to play games with your lives."

- Dieter

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, and can use the Doppelganger Captain Command.

He is first battled in Bowser Path, where he, Kaley, and Beef take on Bowser Jr., Ludwig, Kamek, and the four Captains from Mario & Luigi: Superstar Saga + Bowser's Minions.

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.''