Swappers appear as common enemies in the game's first area. A Swapper is never seen in battle without Goombas, Koopa Troopas, or Goomba Troopas, although it does appear on the overworld. In battle, Swapper has two attacks: one is sending small Bob-ombs out of its hatch that Mario and Luigi must jump over. Its other attack is raising a satelitte out of its hatch that creates a bright flash of electricity. When the smoke clears, all of the Goombas have become Koopas and vice versa. This has no effect on Goomba Troopas.A stronger variation called Swapper-EX appears later in the game during Neo Bowser City. These Swappers are red and can switch the roles of Fire and Boomerang Bros. at will. They can also attack with small green Hyper Bob-ombs, and summon regular Bob-ombs to the battlefield.
Being mass-produced in a factory, they have very little personality. However, as indicated by their interactions with Bowser, they dislike being referred to as "tissue boxes".