Cheat List:

To enter the following cheats, go to the Cheats screen from the Options menu.
Complete game as Abby: X, R, X, Y, X, Y, R, L
Complete game as Connor: X, Y, L, Y, X, Y, L, R
Unlock Bubble Town Day Robots: Y, Y, Z, Y, Y, R, L
Unlock Bubble Town Night Robots: Y, L, Y, Y, X, L, Z
Unlock Construction Robots: Y, Y, Y, Y, Z, A, R
Unlock Dr., K's Robots: Z, A, Y, Y, Y, X, A