Various Cheats:
Go to the main menu and press and HOLD the SHIFT key and type in MENSAAAA. You should hear "that's what I need". Now put in one of the following codes

Left Shift + Left CTRL + R - Restart level

G - Invincibility:
Left Shift + Left Ctrl + A - Skip Level

Left Shift + Left Ctrl + C - Continue in mission failed

Be Neon's Skeleton:
Go to Neon's profile in the character menu and type the code ZIOCLETO . Now in a mission you play as Neon's skeleton