I-Ninja - Platform: Gamecube

Level Skip:
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While playing, hold R and press B, B, B, X. Release R then 
hold L and press Y, Y. Release L then hold R and press B, B.

Sword Upgrade:
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While playing, press pause then hold L + R and press 
X, B, X, Y, Y, B, X, B.

Big Heads:
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While playing, hold R and press Y, Y, Y, Y. 
Release R then hold L and press Y, Y. 
Release L then hold R + L and press Y, X, Y.

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