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