Ice Nine [GBA]

Cheat mode
Complete the first fifteen missions under the "Graduate" difficulty setting. A cheat menu with unlimited health, unlimited ammunition, mission select option will now be available.

Bonus missions
Complete the first fifteen missions under a difficulty setting greater than "Initiate" to unlock three bonus missions.

Cheat menu (unlimited health and ammo, mission select)
Beat 1st 15 missions on Graduate. Go to "Start Game" to activate cheat menu and play any mission with cheats.

Open doors
When reaching a door or gate that requires a switch to open, hold L + R + Select until the screen flashes the phrase "Trust no one" is spoken.

Veteran difficulty
Complete the first fifteen missions under the "Graduate" difficulty setting.