In Story mode, select any Ultra besides Mebius. After Zoffy is injured, Ultra Mother will tell you that he needs three days to recover. A set of training battles against the five other Ultras will begin. Defeat one and start the second training battle. Immediately press Start when the second training battle begins. Save the game, then quit the game at the menu. Enter the PSP system menu. Advance the date three days forward. Start the game and continue Story mode. Defeat the four remaining Ultras in the training battles. Complete Story mode to unlock Ultra Father.
