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in sonic adventure dx you can only unlock metal sonic by collecting 130 sonic emblems.
You can play as him by not losing a single match and beating Metal Sonic.
It's pretty hard. Select Sonic and play through the game without losing a single match until you meet Metal Sonic. Beat Metal Sonic on the first round. At any time during the second round, initiate hyper-mode and Sonic will become Super.You must defeat all of your opponents without losing once. Then, when you fight Metal Sonic (round 2), activate hyper mode and he will become Super Sonic.
Never lose a single match and beat Metal Sonic once and during the second round Sonic will go super.
Metal Sonic doesn't get a story in Sonic Adventure however you can play him in trial mode to unlock him get all the emblems you can go to every level Sonic goes to as Metal Sonic you can even fight the bosses and play the minigames.