Warning: spoilers ahead for Episode 17 of Gotham Season 4, called "Mandatory Brunch Meeting."
It's always been inevitable that the Joker would turn up at some point on Gotham. Surely Bruce Wayne's origin story would feature the future Clown Prince of Crime! Jerome Valeska, with his maniacal laugh, flair for theatrical violence, and scarred face, has seemed like a prime candidate for the Joker, if not for a number of people on Gotham denying that he's the Joker. Instead, Jerome was described as the "proto-Joker" whose psychotic actions would ultimately inspire the real bad guy. Cameron Monaghan was Joker-esque as Jerome, but he wasn't playing the Joker...or so we thought.
In "Mandatory Brunch Meeting," Gotham introduced Cameron Monaghan as Xander Wilde, a.k.a. Jerome's long-lost and heretofore unknown twin brother, a.k.a.the guy who could very well be the Joker.
Excuse me for a moment while I freak out at the sheer awesomeness of this development! Okay, would it be slightly predictable that Jerome could kinda sorta still be the Joker because he had an evil twin? Sure. Would it also epic if it turns out that Gotham fully embraced the crazy and found a possible way for Cameron Monaghan with that fantastically terrifying laugh of his to stay on as the Joker? ABSOLUTELY. Here's how it happened.
After his breakout in last week's episode, Jerome assembled a Legion of Horribles and decided to paint the town crazy with the help of Scarecrow, Firefly, Victor Friese, and (an admittedly reluctant) Penguin. Jerome wasn't totally off-the-wall in the episode, however, as he had a plan to track down his twin brother, who was hiding in an underground labyrinth to avoid Jerome. At first, Xander (or Jeremiah) seemed smart for hiding from his psycho brother. On second consideration, can a man who literally constructs a massive labyrinth underground with a holding room for his brother really be 100% sane? Like Jerome, maybe he really only needs one bad day.
Anyway, after fisticuffs between Jim and Harvey and most of Jerome's Legion of Horribles, the brothers faced off. Jerome indeed dropped the immortal Killing Joke-esque line about "one bad day" and promised to drive Xander crazy before killing him. In their face-off, Jerome revealed that Xander had claimed that Jerome tried to kill him as kids, which wasn't exactly true. Sure, Jerome may have wanted to take his brother out, but he never tried. Xander ran away, and his claims that his brother tried to kill him ruined Jerome's life and turned their mother against him, resulting in Jerome's "one bad day" and the ultimate murder of their mother. Jerome and his Horribles took off to put their plan to drive the city crazy into motion, and Jim took Xander to the GCPD to put him into protective custody. As icing on the cake, Xander said this as he and Jim parted ways:
Yeah, that might come back to haunt Jim just a little bit if Xander really does turn out to be the Joker, although there are no guarantees at this point. Despite the fact that Xander is clearly not the most sane man in Gotham, he's not quite as off-the-rocker as we'd expect of the Joker. The big transformation into the Joker could happen thanks to the drug that Joker had Scarecrow develop.
The final moments of the episode showed Jerome exposing a businessman to the gas, which resulted in him laughing hysterically with the edges of his mouth twisting up into a grotesque grin before he seemingly died. The Horribles were clearly thrilled at the development, which did prove how Jerome intends to drive Gothamites crazy AND kill them. Perhaps Xander will become the Joker after somehow surviving Scarecrow's drug. He seems to be one bad day away from snapping and going full bonkers; surviving the fear gas could be exactly what breaks him and transforms him into the Joker.
Or the random businessman or another random person in Gotham will be the Joker. Who knows what Gotham will do? We can only wait and see. My fingers are crossed for Xander the Joker.