Hint [Submitted by:Earl ]
In the first confrontation with the Knight, he is pretty hard to kill in the Hard mode, but you can recharge your devil gauges (with that your vitality gauge) by not following him when teleports to the upper levels of the courtyard and shooting him with the grenade gun. He will block your shots but your devil gauge will charge up. You have to play around to find a good vantage point in the courtyard to do this.

Easy mode
Successfully complete missions 1, 2, or 3 while using at least one continue. At the start of the next mission, there will be an option to change the game to easy mode. Note: Once easy mode is activated you cannot return to normal mode.

Hard mode
Successfully complete the game once to unlock Hard mode.

Legendary Dark Night mode
Successfully complete Hard mode to unlock Legendary Dark Night mode.

Dante Must Die mode
Successfully complete Legendary Dark Night mode to unlock Dante Must Die mode.

Super Dante mode
Successfully complete Dante Must Die mode to unlock Super Dante mode. In this mode, you can transform into a devil, and use magic indefinitely.

All-star group photo
Successfully complete the game with an "S" rank on all missions to unlock the all-star group photo.