Digimon Rumble Arena

Impmon:
Successfully complete the game with all three season three characters.

Get Mega form:
To receive anyone's Mega form, successfully complete the single player game with that person.
You cannot be a Mega in single player mode.

MegaGargomon:
Enter MINNYN as a password.

Get digivolved form:
To receive anyone's digivolved form, successfully complete the game with them.

WarGreymon:
Enter QRIOUS as a password.

Sakuyamon:
Enter KENSAN as a password.
Alternately, successfully complete the game with Renamon.

Reapermon:
Successfully complete the game once.

Omnimon:
Enter SERIUS as a password.
Alternately, successfully complete the game with Gabumon and Agumon.

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.