When you beat Roof levels different Minis will become available for the main game or the Construction Zone. To use them in the main game go to 'Options' and select 'Showroom' and tap 'Character' until the one you want is selected.
Unlock Mini Toad: Beat Rooms R-1, R-2, and R-3.
Unlock Mini Peach: Beat Rooms R-4, R-5, and R-6.
Unlock Mini Donkey Kong: Beat Rooms R-7, R-8, and R-9.