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These are the charecters you can Unlock

Venom
Green Goblin
New Goblin
Iron Fist
Doc Ock
Sandman
Rhino
Prowler (already unlocked when game begins)
Blackcat
Blade
Lizard
Thats all I can remember at present time!!!

there's also carnage and electro in the psp version and silver sable in the wii version