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, (likely because Dieter is a German name) including telling Bowser Jr. "auf wiedersehen" (farewell) before trying to kill him.

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

After this, he is next fought alongside the other two Best Fitness Friends members in the final battle, and merges with them to form the final boss before being defeated for good.