Complete the indicated task to unlock the corresponding character.

Cream: Get Knuckles on your team. Go to Cream at the center of the Green Hill Zone. Find Cheese in the tower to the southeast and Cream will join your team.
E-123 Omega: Complete the side quest in the northeast of Metropoplis. Ascend to the top and have Tails fly to the west. E-123 Omega will be in an underground room.