10 Worst Things Harley Quinn Has Done To The Joker
9. Killing His Replacement - Harley Quinn (Vol. 3) #14
After Harley finally left the Joker, it's safe to say that the villain realised quickly what a mistake he had made. Not because he loved Harley, per se, but because she was usually the one breaking him out of Arkham Asylum, and now his escapes would likely take far longer.
To occupy himself in this time, the Clown Prince of Crime trains a man named Edwin, who writes the Joker to tell him he's fallen in love with Harley - you know, like an idiot.
And so the Joker plans the ultimate prank against Harley, setting Edwin up so he looks, speaks, and acts like the purple and green murder machine. It works so well that Quinn legitimately believes Edwin is the clown - right up until she has to change his clothes when he's in a traffic accident. Harley's worries about Edwin's misplaced scars are confirmed when she manages to notice the one difference between him and the actual Joker: their genitals.
Instead of the prank the Joker was hoping for, Harley tortures Edwin and then kills him - making all of his training a big old waste of time.