When you have successfully completed the game 'Save' it and then when you load a new game with the clear game data you will start the game with your previous level, equipment, gameplay time, your demon compendium, maxed Social Links and access to the Monad Block. You will also be able to fight an optional boss and any consumable items in your inventory will disappear.
Start a new game with your completed game detailsAdded 24 Aug 2006, ID #22944
After you have completed the game, you can restart with your current levels, money and other stuff. To do this, just save your completed game, then load it up again and you should be able to start with all your gear.