Unlock Extra Agumon:
Beat the game with Gabumon

Lopmon:
Beat the game with any 3 Digimon but without using the two you used to get Black Agumon

BlackAgumon/BlackWarGreymon:
Using two different characters beat the game twice

Unlock Impmon:
Beat the game defeating Impmon with 300 or more D-Spirit

Lopmon/Kerpymon:
Using any three characters but not the ones used for getting BlackAugmon beat the game. Almost identical to Terriermon in moves.

Gabumon/Omnimon:
Using all the original Digimon complete the game along with Lopmon and BlackAgumon