Jeremiah Valeska

Jeremiah Valeska, also known as Xander WIlde, is a supporting character turned into the final antagonist in the fourth season of the TV series Gotham. He is Jerome Valeska's twin brother and is the show's version of The Joker.

Past
Jeremiah and Jerome were identical twins, and one day, their mom sent them both away, knowing that Jerome will be evil. Jeremiah becomes a very high skilled engineer during life, and he has a proxy working for him. He also changes his identity to Xander Wilde, in order to protect himself from Jerome.

Escaping Jerome Valeska
Jeremiah later sends his proxy Ecco to capture Jerome so he can be held inside a cage. Jim Gordon and Harvey Bullock ask him to hand Jerome to the GCPD, but Jeremiah refuses. Jerome however hypnotizes the proxy and sends her to kill Jim Gordon and Harvey and to bring Jeremiah to Jerome. Jeremiah is taken to his brother, who confronts him with Jervis Tetch and Jonathan Crane, and tells him that he will turn insane and then die. Jim Gordon however starts shooting Jerome, thus chasing him away, and saving Jeremiah.

The New Joker
Jerome later kidnapps Jeremiah and tries again to convince him that he is mad. Jerome gives Jeremiah a knife and asks Jeremiah to kill him. Jeremiah tries to do so, but is kicked on the head by Jerome, who then tries to kill him. Later, Jerome dies, and Jeremiah thanks Bruce Wayne, Jim and Harvey for helping him save the city. He gets a gift from The Wayne Enterprises, which really contains one of Jerome's traps. The trap sprays tons of laughing gas on Jeremiah, who is now brainwashed by Jerome, to become The Joker and have some "fun".