Seraphic Gate
Save the game at the last save point. The Seraphic Gate will now appear on the world map. Note: After completing it you can repeat it with stronger enemies.

Hard difficulty
Successfully complete the game then start a new game. The new game will increase in difficulty, depending on the amount of times the game has been previously been completed. This is indicated by the number and size of the crystals that appear on the main screen.

Original Valkyrie Profile battle music
Successfully complete the game. Load the cleared game file and start a new game. Hold L1 while entering a battle to change the background music to that from the original Valkyrie Profile.

Two player mode
To play along with a partner in the battle, press Select to go to the menu. Go to "Party configuration". In the box where you can change the characters position, press Triangle on the character that player two should control.

Random battle music
Successfully complete the game. Load the cleared game file and start a new game. Hold L2 while entering a battle.

Easy experience
After defeating the Ballistic Rhino at the end of Royal Underground Path, you can fight it repeatedly. When prompted, head into the altar. You only get 1600+ experience points and 290 OTH, but you can also get a Baraka (an accessory that increases a character's HP).

Bonus characters
Defeat the indicated opponent in the Seraphic Gate to unlock the corresponding character.

Dylan: Defeat Gabriel Celestia on floor 1.
Freya: Defeat Freya on floor 4.
Hrist: Defeat Woden on floor 2.
Lenneth: Defeat Woden on floor 2.
Leone: Defeat Gabriel Celestia on floor 1.
Lezard: Defeat Obsessed Ex on floor 3.
Silmeria: Defeat Woden on floor 2.
Valkyrie: Defeat Ethereal Queen on floor 5.