you can't the only super form of sonic is super sonic and if you really want dark SUPER sonic you have to cheat
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in sonic adventure dx you can only unlock metal sonic by collecting 130 sonic emblems.
In Sonic Adventure 2 Battle(she is very hard 2 unlock get all rank A's with sonic!)Sonic Adventure DX,Sonic Advance 1 2 and 3(in 1 of them you have 2 unlock her).Sonic Riders.
You have to get 100% on all the characters stories. (Note: Sonic has to go through ten levels while everyone else has to go through five)
At here www.sonicroms.net.Go to roms & games select sonic and chose sonic adventure dx.
To unlock the Last story on Sonic Heroes, you have to play all other stories:team sonic, dark, rose and chaotix all the way through (100%)