Access all characters:
At the Opening screen and before the Banpresto logo disappears, press L (9 times), R (7 times), Z, C-Down (10 times), and C-Right (5 times).

Play as Master Gundam:
Beat Master Gundam three times in Story mode.

Play as Devil Gundam:
Beat Story mode with every character. Then start 64 Mode. Choose Master Gundam and beat Devil Gundam to unlock the Devil.

Play as Judecca:
Play the game for 300 hours to unlock Judecca.

New outfits for Dunbine and Walker:
Play the game for 50 hours. Now highlight either character and press Start.

Thanks to Revolution readers Kevin and El Dopa!