Successfully complete the game with Sonic, Knuckles, Silver, and Shadow
to unlock Metal Sonic.

To unlock Sonic's leather suit, get the Sonic The Hedgehog, Sonic The
Hedgehog 2, and Sonic The Hedgehog 3 cards. To unlock the gloves, get
the Cream The Rabbit, Cheese, and Cream and Cheese cards. To unlock
the shoes, get the Sonic and Amy, Amy Rose 2004, and Amy Rose 2005 cards.

Successfully complete the indicated task to unlock the character skin:

Knuckles - Successfully complete Story mode with Knuckles.

Metal Sonic - Successfully complete Story mode with Metal Sonic.

Shadow - Successfully complete Story mode with Shadow.

Silver - Successfully complete Story mode with Silver.

Sonic - Successfully complete Story mode with Sonic.