If you chose Agumon or Guilmon in the beginning, go on the Ghost Extermination quest at the Shrine of Seals and sacrifice the "Defense ROM Delta". This must be done on Very Hard mode. If you chose Veemon or Dorumon in the beginning, you can get him by going on the Booster Pack Collection sidequest set by the Digi-Elfs in the Terminal Area and completing the dungeon with only 1 HP, without recovering your HP or using a Gate Disk. This must be done on Very Hard mode.