I-Ninja Cheat Codes for Gamecube

Unlock Battle Arena
To Unlock Battle Arena, earn all the Grades, Then you should get the Master Grade. The Battle Arena Will be open at Robot Beach.

Level skip
Press Start to pause game play, then hold R and press Bx3, X. Release R, then hold L and press Yx2. Release L, then hold and press R, Bx2.
Submitted by: Dick

Upgrade sword
Press Start to pause game play, then hold L + R and press X, B, X, Yx2, B, X, B.
Repeat this code three, four, or five times to get Soul Sword.
Submitted by: Dick

Big head mode
Press Start to pause game play, then hold R and press Yx4. Release R, then hold L and press Yx2. Hold R + L, then press Y, X, Y

Mow grass
Where there are grass, flowers, mushrooms, other foliage, objects other than crates and barrels, and certain decorative objects, you can cut them down to earn a few extra points towards your main score. This is useful in the certain levels that require a high score to earn the grade.

xPower-Ups
Whenever you kill an enemy in a rapid succession, the number to the right of the health bar increases by one up to 10. This represents what points you receive for killing an enemy, multiplied by the number displayed. This is useful in certain missions where a high score is required to earn the grade.
Submitted by: Dick

Healing
If you are low on health, try to find some weeds and chop them down. Notice that your Rage Ability switch will gradually increase. Keep chopping down weeds and you will soon be able to use your "Ninja Revive" to heal yourself.

Get stronger swords
To get stronger swords, you have to kill enemies. Notice that when you complete a mission (at the blue screen with Ninja floating), there is a sword at the bottom of the screen called "Ranx Bank". The sword has numbers that increase with the more "Ranx" that you kill. Also, the more "Ranx" you kill, the sword gets covered in Ranx blood. When the sword is full of Ranx blood, at the "Mission Complete" screen, Ninja will be holding his old sword. He will drop the old sword and a new one will fall into his hands.