Asagi mode:
To unlock Asagi mode, at the title screen, highlight the “New Game” option, and press Triangle, Square, Circle, Triangle, Square, Circle, X. In the Japanese version, press Triangle, Square, X, Triangle, Square, X, Circle. Alternately, collect all the various Torn Letters hidden throughout the game.

Commit suicide:
To make Prinny explode and restart from the last save point, during gameplay, hold L R and press X.

Cave Of Ordeals stage:
To unlock the Cave Of Ordeals stage, collect 70 Lucky.

Fight against Etna:
To fight against Etna, collect 100 Lucky, then select the third option in the castle door.

Prinny raid skill:
Lose all 1,000 lives, then start a new game.

Alternate endings:
Unlock the following alternate endings by performing the correspondign tasks:

Asagi – Successfully complete Asagi mode.
News Of An Exploding Penguin – Lose all 1,000 lives.
The Escape Of The Prinny – Talk to the ninja, and select the first option.

Lose 1,000 lives easily:
Use the following trick to unlock the Prinny raid skill easily. Restart the game by going to the Runaway Shop. Select the “Hell’s Finest” difficulty. Choose the Moab Fortress, and proceed until reaching the Boss. Allow the game to idle here to lose 1,000 lives in approximately two hours.

Defeating Chefbot 9000:
To defeat Chefbot 9000, stun him, then attack him. After the stars fade above his head, hip-pound the flaming keyhole, and his shield counters will fade.

Defeating Prinny Laharl:
To defeat Prinny Laharl, stun him, then do not attack him further. Pick him up, and throw him to instantly kill him.