Play as Gaia:
To play as Gaia, at the title screen when the words come sliding from the sides onto the screen, press Down, Down + Left, Left and hit Weak slash. Weak slash is the Square button by default. You will hear the word "FIGHT!" confirming the code worked. Now on the character select screen, hold up on Eiji and hit any attack button. You can now play as Gaia.
Play as Sho:
First put in the Gaia code, let the game go to the demo, and press start on controller 2 to go back to the title screen. As the words come in, press on controller 2: L,R,L,R,L + Square. You should hear "fight", and the text will turn blue. Then go to player select screen, go to Kayin, push down and square to select Sho.
Instant Secret Moves:
After entering the codes to play as Gaia and Sho, wait through the demo until the title screen comes up yet again. Like before, as the options come in from the left, motion Down-Left + Weak Kick on controller 1. You'll hear "Fantastic!" and the option text will turn white. To utilize this feature change the control type to use L1, L2, R1, R2 as SPECIAL buttons. This can only be done on Easy or Very Easy. Now anytime during a fight hitting L1 + L2 + R1 + R2 + Select at the same time will unleash the character's Secret Move.
After entering the Instant Secret Moves code, wait for the demo to start. Press Start, and when the title screen appears and the text is scrolling in from the side, press: L, R, L, R, L + Square on controller 2. If done correctly, you will hear the announcer say "Fantastic!" and the text on screen will be yellow. This code allows you to use any control configuration on any difficulty.