BlackWarGreymon
Play the game under any difficulty setting without losing a round (not a continue). During the fight with the last Digimon before Reapermon, BlackWarGreymon will challenge you. Defeat him and Reapermon without losing a round to unlock him.
Gallantmon
Enter KIMJOY as a password.
Imperialdramon
Enter LINMON as a password.
Impmon
Successfully complete the game with all three season three characters.
MegaGargomon
Enter MINNYN as a password.
Omnimon
Enter SERIUS as a password. Alternately, successfully complete the game with Gabumon and Agumon.
Reapermon
Successfully complete the game once.
Sakuyamon
Enter KENSAN as a password. Alternately, successfully complete the game with Renamon.
Special Digimon
Go to password at the main menu, and put in ROYBOY to get a mega digimon--WarGreymon or BlackWarGreymon!
WarGreymon
Enter QRIOUS as a password.