Successfully complete the indicated task in Planet Superstars mode to unlock the 
corresponding character:

Alex Kidd: Successfully complete Tournament Singles in the "Alex Kidd" stage. 
Amy Rose: Successfully complete Mission 2 in the "Sonic The Hedgehog" stage. 
Gilius Thunderhead: Successfully complete Mission 1 in the "Golden Axe" stage. 
Gum: Successfully complete Mission 12 in the "Jet Set Radio" stage. 
MeeMee: Successfully complete Mission 8 in the "Super Monkey Ball" stage. 
Pudding: Successfully complete Mission 1 in the "Space Channel 5" stage. 
Reala: Successfully complete Mission 2 in the "NiGHTS" stage. 
Shadow the Hedgehog: Successfully complete Mission 14 in the "Sonic The 
Hedgehog" stage.