Unlock versus mode characters:

To unlock...       ...you have to: 
Big The Cat        Buy all pictures of Tails in Sonic's room 
Chao               Buy all pictures of Knuckles in Sonic's room 
E-102 Gamma        Buy the last picture of Amy in Sonic's room 
Super Sonic        Buy all pictures of Sonic in Sonic's room 
Bonus Characters   Beat story mode 

Hidden artwork:
     Put the disc in you PC CD-ROM and you will find hidden artwork.