Best Fitness Friends

The Best Fitness Friends are the main antagonists of Bowser Jr.'s Journey from the 2019 3DS remake of Mario & Luigi: Bowser's Inside Story. They are a group of humanoid mercenaries hired by Fawful to trick the Koopalings and Bowser Jr. into believing they are fitness gurus with a cure for the Blorbs. In actually, they are tricking them to leave Bowser's Castle so Fawful can take over. During the final battle, the three merge together to create a monstrosity that serves as the mode's final boss.

Biography
The Best Fitness Friends first appear when Fawful introduces them to Midbus outside of Bowser's Castle. After Bowser flies off to integrate the Blorbs meeting at Princess Peach's Castle, the trio enter the castle and show off their special cure for the Goombas infected by the Blorbs. While the Koopalings are infatuated by their technology, Bowser Jr. is not impressed and goes off to find his own cure for the Blorbs.

While he is away, the Best Fitness Friends reveal their loyalty to Fawful and imprison Bowser's men, while also brainwashing others into serving Fawful. The Koopalings realize this and ally with Bowser Jr., although Morton is brainwashed temporally. Once Bowser Jr. obtains all of the ingredients for the Blorb antidote, Kaley steals them, enraging Bowser Jr. to return to Bowser's Castle.

When Bowser Jr. makes it to his father's castle, Fawful had already left to steal Peach's castle and left the Best Fitness Friends in charge. Bowser Jr., the Koopalings, and their army fight through the BFF's henchmen and encounter them for a final battle. Following their first defeat, the trio fuse into a giant monster that creates shadowy copies of the Koopalings. It also swallows Bowser Jr.'s antidote to increase its stats. Once it is defeated, it spits out the antidote and explodes.

Dieter
The sunglasses-wearing leader of the group, Dieter is the leanest and meanest. He wears a black and yellow suit and has yellow hair. His attacks involve throwing cards, and has the ability to create duplicates of himself when his army is low on numbers.

Beef
Beef is a martial artist and the main powerhouse for the group. He has a blobish appearance with red hair and black pants with a red karate belt. While his speed is the slowest out of the trio, his power goes beyond Dieter and Kaley.

Kaley
Kaley is a small blue hair girl who can flap in the air like a bird. Her attacks involve producing shockwaves by shrieks.

Trivia

 * The Japanese names of Dieter, Beef, and Kaley (Sulimu, Muchio, and Kyutto) are alterations of "Slim", "Macho", and "Cute".