First rescue the Titanium Ranger in Level 7 (final level) and then finish the rest of the game. Wait for the credits to finish and save the data again. Load up and start a new game and the secret character you've couldn't get on the beginning of the game will be unlocked. Make sure you load it when you wanted to play the game again.
Beat the game. Flash through the credits and Mrs. Fairwether will come up and say how to. Press Z on certain levels and the screen will say "Secret Bonus Level. It's basically the same level but something will be messed up. On "eye in the sky" the enemies are huge.
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