9) She Was Only Meant To Appear Once
Originally, there was no plan to give the Joker a significant other, and Harley’s creation came about just by chance. In BTAS episode “Joker’s Favor,” Dini and Timm thought they needed a female henchman for the Clown Prince of Crime.
In fact, the only reason they needed one was because they wanted someone in female clothing to jump out of a cake for a quick gag. Feeling that cross-dressing wasn’t in the Joker’s bag of tricks, they came up with Harley for that one small scene. If they had stuck to the original plan, she would never have appeared again.
Thankfully, the fans immediately took to the character and the potential therein. Naturally, Dini and Timm listened to their viewers and Harley soon became the romantic foil to Mr. J that she is today.