He decides to become a Batman Villain

Robby decided to merge with one of the villains in Batman's Rogues Gallery. He could become that villain and have access to the powers of the villain and the power of the key. That was enough to control Batman... But which one to take?

The most common choice was the famous Joker. As such he would have chemical knowledge and science skills, plus a pension for doing the unpredictable. He would also have the advantage of having Harley Quinn as his henchwoman. A sexy babe in her own right.

Or he could become the Mad Hatter. Master of mind control and nanotech, he had the power to control the minds of others. It would give him an edge with the power of the key backing him up.

Or he could become Two-face. The deformed villain with the split personality and a gimmick for twos and luck. A flip of the coin would decide things for him, influenced by the key.

Or he could try one of the other villains that lined Batman's Rogues Gallery. He could gain their power to use with his own.

What does he choose?