Unlock the following characters by performing the corresponding tasks:
CHARACTER - HOW TO UNLOCK Cream - Get Knuckles on your team, then go to Cream at the center of 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.
To easily complete Kron Colony, defeat the first Kron you encounter. Save the game, then go south to an opening. You will fight another Kron. Defeat him, then with Amy in your team, go to the opening west from that area. Once you enter, bash the boxes with Amy's hammer. Go forward, and ignore the Kron. If you get attacked by him, just run. Go to the next room. Continue until you reach the ruler, then speak with him.