Young Bruce Wayne actor David Mazouz told fans at the Heroes & Villains Fan Fest in London over the weekend that fan favorite Harley Quinn would be making her debut during the 2-hour Gotham season finale. There had been busy about the character joining the series since Jerome Valeska (Cameron Monaghan) returned with a sew-on face making him the Joker in everything but name. In April we looked at the possibility of bringing Harley Quinn in and the problem with the age difference. Also the fan theory that Barbara Kean (Erin Richards) is going to become Harley.
Even with how crazy things have become on the series, the villains become the villains. Edward Nygma has become the Riddler, Oswald Cobblepot the Penguin, Victor Fries is now Mr. Freeze, etc. Only characters like Jerome and Captain Nathaniel Barnes were unique but became known villains, but that's because their real names aren't known in the comics. It's hard to imagine they would take a character that is supposed to be Jim Gordon's wife and mother of Barbara Gordon and turn her into Harley Quinn.
I think what is more likely is that since Jerome isn't an active part of the current plot line, we will 'check-in' with him in some way during the finale and find that he is being treated by a Dr. Quinzel. Or something to that effect. I can't see the writers of the series not stretching the characters origin out of a few episodes or more. Do some really fun scene between pre-Harley Harley and Jerome.
The 2-hour Gotham season finale airs Monday at 8 PM on Fox.