Monaghan spent multiple seasons on Gotham portraying Jerome, the over-the-top, pseudo-Joker that always managed to insert chaos into the city. However, things changed at the end of Season 4, when Jerome was killed and his twin brother Jeremiah was revealed to be a much more cruel, intelligent, calculated killer.
During an interview with EW, Monaghan was asked about his portrayal of Jeremiah and Jerome over the years, and the actor took this opportunity to tease the mysterious third role.
"It’s a very strange and surreal thing but on a show that’s as strange and absurd and heightened as Gotham, it makes sense. It’s a weird challenge," Monaghan said. "It’s definitely not what I expected going into it, to develop one character for as long as I did and be like, all right, time for a new thing that’s also distinct. And then I’ve had to do that again recently in the final season as well — I won’t say how or why but there’s another huge transition for the character too. It’s definitely an extremely unique challenge that’s unique to the comic book medium."
Later on in the interview, Monaghan is asked about playing both Jeremiah and Jerome, and which role he had more fun with. In his response, the actor once again brought up his third role, saying that it was the best of the bunch.
"They’re both a complete joy," he began. "I’ve had a lot of fun with Jeremiah in season 5. Because Jeremiah and Jerome were on screen at the same time in season 4, I had to take make them extremely distinct. So I had to take the extreme of Jeremiah of making him much more stoic. Now with Jerome out of the picture I get to make Jeremiah much more heightened in his own unique way. He’s eccentric as like a David Bowie type. He’s foppish a little bit. I took a lot of inspiration from Tim Curry in Rocky Horror Picture Show. He’s this eccentric, flamboyant, arrogant character. But the character I most enjoyed playing is the one in the final episode that we haven’t seen yet."
There are no details regarding this third role as of yet, but it's likely another form of the Joker, perhaps even the final version that will be able to take on the iconic villain's name. As we know, Batman will appear in full during the series finale, so it would make sense for the fully-realized Joker to arrive as well.
New episodes of Gotham air on Thursday nights at 8 pm ET on FOX.