Agumon
An extra comes with Gabumon. He is the same as the first one, only with a different digivolution and can float.
Black Agumon and Black WarGreymon
Successfully complete the game two times with two different characters.
Gabumon and Omnimon
Successfully complete game with all default Digimon, Lopmon, and Black Agumon.
Impmon
Complete the game with 300 or more D-Spirit, and having defeated Impmon.
Lopmon and Kerpymon
Successfully complete the game with three characters (except the ones used to get Black Agumon.) to unlock Lopmon.