If you want to play as the bosses do the following, beat the game on four,
then go to player select screen and
go to the question mark, press and hold down the select button and you will 
notice that Master and Uranus are
playable press triangle to play as any of them.

To play all four bosses, beat the game on level 4 to obtain Master and 
Uranus; go to "?" and hold down "select" to choose. Then change to level 5, 
beat the game again and now you can choose Sho and Vermillion.

To Get all of the bosses unlocked without having to play thru the game on the 
first title screen (when the letters for Takara scroll) type in L1, L2, (Triangle), 
R1, R2, (Square) on Controller 1and that will Master & Uranus for Vermillion 
& Other Chracters type in (Square), R2, R1, (Triangle) L2, L1.