Legendary Dark Night mode
Complete Hard mode to unlock Legendary Dark Night mode.

Dante Must Die mode
Complete Legendary Dark Night mode to unlock Dante Must Die mode.

Easy mode
Complete missions 1, 2, or 3 while using at least one continue. When you then start the next mission, there will be an option to change the game to easy mode. Just remember that when easy mode is activated you cannot return to normal mode.

Super Dante mode
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
Complete the game with an "S" rank on all missions and you will unlock the all-star group photo.

Demon form
Defeat Dante Must Die Mode to unlock infinite Demon form.

Collecting red orbs
When you have killed all the enemies in a room go through a door and then go back in, now your opponents will be back so you can kill them again to collect more red orbs. If you kill them using only your sword instead of a firearm, you will be reworded with more orbs.

In hard mode when you face the phantom spider for the second time during mission 4, in corridor where he chases you, kill him instead of escaping through the door on your left. Make sure you have your devil trigger fully charged. You will get 500 Orbs after killing him. Save the game, restart the level, and repeat.

10 hit combo
Press O twice then wait for 1-2 seconds and then press it again. Dante should then perform a combo of about 10 hits.

Skip FMV sequences
Press Select during an FMV sequence.

Hint: Quick transformation
To do this trick you must first get Ifrit. Quick change between Alastor and Ifrit by pressing R3 (R3 = the control stick).
Hint: Quick Devil Trigger
When surrounded by enemies press R2 to taunt the opponent. The Devil Trigger meter should go up by one.