The Best Fitness Friends, later renamed the Brute Force Federation, 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 who serve Fawful in his attempt to conquer the Mushroom Kingdom. They trick the Koopa Troop under the impersonation as fitness gurus with a cure for the Blorbs into leaving Bowser's Castle unguarded, so Fawful can take over. In actuality, they plan to betray Fawful and claim the Mushroom Kingdom for themselves. During the final battle, the three merge together to create a monstrosity that serves as the mode's final boss.
The BFF 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 enters his castle announcing themselves as the Best Fitness Friends. Although they are almost kicked out by Roy, they fascinate him and the other Koopalings by showing their special cure for the Blorbs called Skeletone Formula:D, which they demonstrate by curing a Goomba of its Blorbs. They tell the Koopalings that they have no more and offer to make more if they gather the ingredients. Bowser Jr. boasts that he can find them and goes off with the Koopalings to find the ingredients.
This turns out to be a ploy for the Koopalings to leave Bowser's Castle unguarded, allowing Fawful to easily take over the castle. During Bowser Jr.'s quest, the members of the Best Fitness Friends appear and encourage Bowser Jr. to keep going. But they also manipulate the Koopalings into separating from Bowser Jr.'s party, either by making them turn against him or by causing a situation that forces them to split. These include dispatching Ludwig and pushing Iggy off a cliff.
Once Bowser Jr. collects all ingredients for the Skeletone:D and combines them with help from Broque Madame, Kaley steals his Skeletone:D. The BFF reveal their affiliation to Fawful and their true name to be the Brute Force Federation. They combine Jr.'s Skeletone:D with the poisonous breath of a Malodorius Fibbian to create Malatone Formula:X, which they use to brainwash Morton into throwing Bowser Jr. and Kamek into the sea. They also reveal their intentions to use more Malatone:X to brainwash Fawful's army so they can usurp Fawful for control of the Mushroom Kingdom.
Washing onshore Plack Beach, Bowser Jr. realizes how selfish he was and recruits the Koopalings. Once Bowser Jr. and troop arrive at Bowser's Castle, Fawful had already left to steal Peach's Castle, leaving the Brute Force Federation to supervise. At first, the BFF manage to overwhelm Bowser Jr. and his forces, but Captain Goomba and the other captains fight off the BFF's minions, allowing Bowser Jr. to rescue Wendy and Roy, as well as feed Morton a Renewaleaf to cure him from the Malatone:X.
As the world darkens from the presence of the Dark Star, Bowser Jr. and his team fight through the BFF's henchmen and engage against them for a final battle. At first, Dieter attempts to allure Bowser Jr. and asks if he fights for good or evil, to which Junior replies that he fights for his own side. The BFF sent out their remaining minions before facing the team themselves, boosting their strength with Malatone:X. Following their first defeat, the trio fuse together into a giant Malatone-infested monster that creates shadowy copies of the Koopalings. It also swallows more Malatone:X to increase its stats. After a lengthy battle, Bowser Jr. and his troops defeat the monster and reclaim the castle and Skeletone:D.
Dieter is the sunglasses-wearing leader of the group. He is the smartest member of the trio and will do any means to achieve his goals. 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 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. Though he is very powerful, he is quiet and slower than his partners. For a majority of the game, Ludwig was disguised as Beef, as he managed to defeat Beef back at the castle when Beef pushed Ludwig in lava, only to magically teleport behind Beef and push him in, before being locked in the Bob-omb storage room. He only found his way out when Bowser blew up the room.
Kaley is a small girl with blue hair tied in a ponytail who can flap her wing-like hands in the air like a bird. Her attacks involve producing shockwaves through her voice. While at first appearing calm and polite, she is quick to temper and appears to have a drug addiction. Despite this, she does have a soft spot for Beef, claiming that she feels lost without him.
- The Japanese names of Dieter, Beef, and Kaley (Sulimu, Muchio, and Kyutto) are alterations of "Slim", "Macho", and "Cute" respectively.
- Beef is the only member of the BFF to appear in the main story, where he lies unconscious in the Bomb-omb storage room when Bowser accidentally blows it up.
- The trio's physical colors represent their trooper type: Dieter is yellow (Ranged), Beef is red (Melee), and Kaley is blue (Flying).
- Their team name "Brute Force Federation" was Beef's idea. Kaley instead wanted to call it "Cutie Coalition".
- According to a Nintendo Direct interview, the Best Fitness Friends are from the same species as Midbus.