Successfully complete the game with an "S" rank on all missions.
Successfully complete the game under the hard difficulty setting.
Successfully complete all missions with 100 points.
Successfully complete the game, then highlight the "New Game" option at the main menu, hold L and press Start.
Successfully complete the game by only using the Zet Zaber and get an "S" rank.
Successfully complete all missions with 100 points.
Successfully complete the game by only using the Buster Shot and get an "S" rank.
Successfully complete the game after all the hidden discs have been collected. Then highlight the "New Game" option at the main menu, hold R and press Start.

Beat the game to unlock Zero's mini-game. A Mini-games menu should appear at the title screen, and you can select it from there.

Complete all missions with 100 points: Fefnir Mini-game

Use Zet Zaber for the whole game and beat the game with S rank: Harpuia Mini-game

Complete every mission with 100 points: Leviathan Mini-game Use the buster shot for the whole game and beat it with an S rank: Phantom Mini-game

Successfully complete the game, then use your cleared saved file to complete the game a second time.
Successfully complete the game.