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9: The Sewers Hero Mode: Need Purchase of Gold Key & Anti-Toxic Character. In the area where there is lots of toxic goo on the ground, go to the right to find a ledge with a ladder, go down the ladder & right to find a Gold Key Panel, use it to get Joker Card # 17 Tim Drake.
You get the Gold Key by finding the Red Brick in Bat-Copter Chase (Hero or Villain), then you need to buy it in the Batcomputer. You also need a character that can walk through toxic stuff (like Bane for example).