Enter the following as passwords to unlock the respective Digimon:

MINNYN		Megagargomon
LINMON 		Imperialdramon
ROYBOY 		Imperialdramon paladin mode
QRIOUS		Wargreymon
SERIUS 		Omnimon
KIMJOY		Gallantmon
KENSAN		Sakuyamon

Get Reapermon:
Beat the game once.

Get Blackwargreymon:
Beat the game once without losing a round. Blackwargreymon will challenge you. Beat him and then Reapermon without losing and you'll enable him as a playable character.

Get Impmon:
Beat the game with Guilmon, Terriermon and Renamon

Get Beelzemon:
Beat the game with Impmon

Get Metalgarurumon, Seraphimon, Stingmon and Magnadramon:
Beat the game with Gabumon, Patamon, Wormmon and Gatomon.

Younger Tai, Matt, T.K. and Kari:
Press and hold L1 and R1 while choosing their partners.

Thanks to Revolution readers Pavpavo, Greg Antonucci, Santiago Cainelli, Angel, Cyberdramon, Ethan Reimer, Phil M., and james denny!