When you get all 130 emblems you can play as metal sonic on trial.
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You Must View All The Ending Credits For All The Characters
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.
Emblems in Sonic Adventure 2 allow more content to be bought from the Black Market in Chao World, including character menu themes. Collecting all 180 will unlock a 3D version of Green Hill Zone.
get all emblems(levels and fields).
I heard a rumor that you have to acquire all 180 Emblems to earn the special stage. I myself am not attempting that kind of feat.