Password System
To get to the password input screen, go to the main menu of the game. Tap "LEGO Store" to enter the store. There will be a red brick in the store -- tap it to get to the password input screen. Use the virtual keyboard to input the cheat codes.
Character Codes
Enter "UGDRSQP" at the password input screen to unlock the Islander. Input "SHWSDGU" at the password screen to unlock the Ninja Master chracter. Enter "XRCTVYB" to unlock the Troll King, and enter "ZVDNJSU" to unlock the leader of the Space Criminals.
Revealing Minikits
Enter "LJYQRAC" at the password screen to reveal all of the Minikits in the game. Collecting all of the minikits will unlock the remaining characters with the exception of Santa, who can only be unlocked by playing the game on Christmas.
Other Useful Cheat Codes
Although the Reveal Minikits cheat is useful for finding all of the Minikits in any given map, you'll still need to worry about getting attacked by enemies while you search. If you prefer to rush through a level to collect the Minikits, use the Health Regeneration or Invincibility cheats. You also may want to use the Reveal Map cheat to reveal any maps that you haven't fully explored. Enter "ABABLRX" to activate Health Regeneration or "HJCRAWK" to activate Invincibility. Enter "SKQMXPL" to activate the Reveal Map cheat.