Collect A ranks for all missions for the following characters and earn the special characters in 2P mode.

Sonic - Amy Rose
Shadow - Metallix (from Sonic CD)
Knuckles - Tikal (from SA1)
Rougue - Chaos 0 (from SA1)
Tails - Chao Walker
Robotnik - Big the Cat (from SA1)

Earn 180 emblems to unlock a new stage - The classic Green Hill Zone from Sonic's Mega Drive debut.