Even after unlocking all 23 cheat options through the single player campaign, there is still an empty 24th space left in the cheat menu. This space is left for nine cheat options which cannot be unlocked during the course of the game. They can only be added to the menu using a GameShark. The cheat options include:

Extra Characters (800696XX 0001)
Maximum Ammo (80069654 0001)
Line Mode (80069657 0001)
Super 2X Health (80069658 0001)
Super 2X Armor (80069659 0001)
Extra Weapons (8006965D 0001)
Super 10X Health (80069660 0001)
Multiplayer Invisibility (80069666 0001)
Debug Position Display (80069669 0001)

Several of these cheats can also be activated by button codes in-game.