YuYu Hakusho: Dark Tournament Cheats

Enter Square, up, square, circle, down, circle. You will see the unlock everybody light up if you entered the cheat correctly.

Complete dark tournament under easy difficulty

If you have taken damage, back up and perform his regeneration move. Pause the game while still holding the buttons needed for regeneration. As soon as the game is paused, you do not have to hold the buttons anymore. While paused your character will still continue to heal. If you wait long enough, he will return to full health.

The only cheats I've been able to find so far are as follows:
All fighters-Square, Up, Square, Circle, Down, Circle
Infinite Ki (Player 1)-Square, X, Circle, Square, X, Circle
No Damage (Player 1)-Circle, Square, Down, Left, Circle, Square

Select Younger Toguro as your final opponent in arcade mode and win without losing any rounds.

Beat dark tournament episode 6: Survival mode
Beat dark tournament episode 7: Tag team mode
Beat dark tournament episode 13: The cliff
Beat dark tournament episode 21: The cave
Beat dark tournament episode 22: Dark arena
Beat dark tournament episode 30: Pro mode
Beat dark tournament episode 30: Crashed arena
Beat Younger Toguro's arcade mode without losing: Training Dojo