Play As Vermillion:

At the character select screen, put the cursor on the question
 mark, then press and hold R1+R2+L1+L2. If done correct you
will hear her laugh, then just press any button to play as Vermillion.

Easy Bosses:

Play as Bosses: There's an easier way to play as the bosses 
without having to beat the game. At the title screen press
R1,L2,X,L1,R2,O You will hear a chime if done correctly. This 
allows you to play as Uranus and Master by holding the Select
button on the random character square. Next, get back to title 
screen without resetting the Playstation. Then, enter the code
backwards in the same manner (while words are flying in). 
O,R2,L1,X,L2,R1 This allows you to play as Vermillion and Sho.

 -
Ricky D

Final Boss:

Go to the Title Screen and press:

UP, DOWN, UP, DOWN, UP and TRIANGLE

Play as the Bosses:

Firstly you must complete the game on Skill level 4 (battle mode), 
this will release two bosses - URANUS and MASTER. 

Use one of these Bosses to complete the game on level 6 (battle mode). 
You must not use any continues. Two more Bosses
will be released for you to select - SHO and VERMILLION.

To get the Final secret Boss, you must complete the game 
on level eight (battle mode) using VERMILLION, then on the title
screen enter the following:

LEFT, LEFT, RIGHT, R1, TRIANGLE, L2, SQUARE

YOU WILL HERE A BELL IF YOU HAVE DONE THIS CORRECTLY.

Armoured GALA:

You will have to be very quick for this one. Select GALA 
and before it says "FIGHT" quickly enter the following:

UP, UP, UP, DOWN

Five peices of armour will fly around GALA and 
attach to his body for extra protection.