If you chose Agumon in the beginning, then complete Item Road after you complete the game. If you chose Veemon in the beginning, go on the "Ghost Extermination" sidequest in the "Shrine of Seals" and complete the dungeon with more than ten minutes remaining. This must be done on Hard mode. If you chose Guilmon in the beginning, complete the Undead Yard, defeating all 279 enemies within thirty minutes. If you chose Dorumon in the beginning, complete the Undead Yard on Hard mode, killing all 302 enemies and have 10 minutes or more remaining. You also need to activate all the "Concealment Gates".