Complete the indicated task to unlock the corresponding mini in the main game or the Construction Zone. To select them for the main game, go to "Options", then "Showroom", then tap "Character" until the desired mini appears.

Mini	How to unlock
Donkey Kong Mini	Complete Floors 7 through 9 of Roof.
Mini Team (Mario, Toad, Peach, Donkey Kong)	Complete Floors 10 through 12 of Roof.
Peach Mini	Complete Floors 4 through 6 of Roof.
Toad Mini	Complete Floors 1 through 3 of Roof.