- "My brother once said all it takes is one bad day to reduce the sanest man alive to lunacy. Jerome gave me what I thought was the worst day of my life. But only by losing everything was I able to face what was inside me. And I believe I've seen something very special - inside of you as well. But, to free it, you'll have to lose everything and everyone you hold dear."
- —Jeremiah Valeska to Bruce Wayne[src]
Jeremiah Valeska is a mentally deranged and dangerous terrorist, criminal and murderer. He is also the more intelligent and more dangerous identical twin brother of the anarchistic cult leader Jerome Valeska. After his brother's death, Jeremiah takes over the role of main antagonist for the final arc of Season Four.
As a child, Jeremiah supposedly suffered years of abuse at the hands of his psychopathic brother, who even burnt Jeremiah's bed while Jeremiah himself was in it. Jeremiah would in turn manufacture several lies about his brother that would turn Jerome's entire family against him; it remains unclear how much truth there was in either story. In a desperate attempt to defend Jeremiah from his monstrous brother, his uncle took him away from Haly's Circus, in which his mother Lila worked, and proceeded to take him to St. Ignatius. Following this Jeremiah changed his name and started calling himself Xander Wilde. After finishing school, Jeremiah became a brilliant engineer, designing highly advanced generators for Wayne Enterprises. He also built a labyrinth hideout to protect himself from his brother, which took almost six years of construction. In addition to the hideout which he filled with security cameras, rooms of work and more, he met a young woman named Ecco who started to work for him as a proxy and assistant. Jeremiah would later reunite with his brother, who after a rampage to find his twin, found him in his hideout.
Right after witnessing his brother's death, Jeremiah received a gift sent by Jerome, which is revealed to be a "special" gas created just for him. Upon inhaling it, his face became pale white and a wide red grin formed from ear to ear, finally giving Jeremiah the drive to explore that dark side of him that his brother Jerome had seen. Becoming obsessed with both Bruce Wayne (whom he viewed as a kindred spirit) and re-building Gotham in his own image after destroying it, he eventually formed an alliance with Ra's al Ghul. After mortally injuring Selina Kyle in an attempt to destroy Bruce's attachments and drive him insane, Jeremiah killed the mayor and demolished all of Gotham's bridges, cutting the city off from the rest of the world and paving the way for Bruce to become the "Dark Knight" Ra's had foreseen. With Gotham City reduced to a lawless "no-man's land", Jeremiah managed to escape into the night, remaining on the loose in the abandoned city.
Jeremiah's past is full of mysterious contradictions due to different inconsistent stories told by multiple people. For sure is that he was born to Lila Valeska and Paul Cicero along with his twin brother Jerome. When Jeremiah was a kid, he already drew blueprints for mazes, not unnoticed by Jerome.
At least according to Jeremiah, Jerome would abuse and scare Jeremiah when he was little. Jeremiah would later recount that he realized Jerome was dangerous and knew he needed to get away from him so he snuck away one night from Haly's Circus and had his uncle Zachary place him at St. Ignatius. Shortly afterwards, Jeremiah took on the alias of "Xander Wilde". He himself would later explain that he did this since he wanted to make it more difficult for Jerome to track him down.
Jerome on the other hand claimed that Jeremiah only made his mother believe that Jerome tried to kill him and stories about him holding a knife to his brothers throat or trying to light him on fire were made up by Jeremiah. However, Jeremiah would later agree that some of these things didn't happen exactly like that. According to Jerome, Jeremiah was the one who is responsible for his brother's madness by essentially creating him, giving him his 'one bad day'. Jerome stated that Jeremiah got adopted by rich people after Lila hid him away, therefore explaining his changed name.
Certainly is that he would then study at school and become an engineer. Four years after graduating, he started construction on his hideout, an underground maze bunker in a forest on the outskirts of Gotham City. The building took 6 years to complete.
At an unknown point in time, he met a woman named Ecco and hired her as his proxy and assistant.
Reunited With Jerome
Ten years after Jeremiah's graduation and six years after he had finished construction on his secret office, Jerome breaks out of Arkham Asylum and kills their uncle, Zachary at the diner he owns for the location of his brother.
Jeremiah is then confronted by the stunned Captain Jim Gordon and Detective Harvey Bullock. They ask Jeremiah about him and Jerome, he tells them that once Jerome tried to kill him, so he ran away and changed his name to Xander Wilde. They then find out that he‘s holding Jerome captive, so they immediately order Jeremiah to let him go but he refuses to do so. After finding out that the Legion of Horribles is inside the building, and that they hypnotized Ecco, Jeremiah takes Jim and Harvey into the halls of his house, which were built as a maze that he’s the only one he can get out of. The three find Ecco there but, since she was hypnotized by Tetch, she gets into a fight with them, Ecco easily wins and takes Jeremiah to Jerome, but she’s eventually knocked out by Jim. Jeremiah then tries to escape but Jerome finds him. They have a verbal fight, in which Jerome tells he’s disappointed by what Jeremiah told about him, since, in his point of view, all of the things he said were lies. Jeremiah did agree that certain things that he told were in fact exaggerations of the truth. Jerome also tells Jeremiah that he’s going to drive him crazy and then kill him. Jim and Harvey eventually get to them, so Jerome runs away leaving Jeremiah behind. Jeremiah is later taken to the GCPD so that he can be put into protective custody. 
Jerome took the Mayor, his wife, and a police officer hostage. He demanded that Jim bring him, Jeremiah and Bruce Wayne. Jeremiah is hesitant to go at first but later agrees. Jerome straps Jeremiah and Bruce into a chair. Jerome tells the crowd the story of him and Jeremiah and then cuts Jeremiah free. Determined to prove Jeremiah is just like him, Jerome hands Jeremiah the knife and tells him to take his best shot. Jeremiah tries to stab Jerome, who knocks him out with a single punch. Later, Jeremiah approaches Jerome’s body after he fell to his death. As Jeremiah walks away, Bruce asks him to let Wayne Enterprises fund his work. Jeremiah thanks Bruce and shakes his hand.
Birth of a madman
- "Hello, brother. Didn't think you could get rid of me so easily, did you? See, my days were numbered, but you oh, you can continue on for me, be my ultimate revenge! So, calm yourself. Don't fight it. Breathe it in. This gas was a special mixture I had made, just for you. Something to finally set you free. It's time to have some fun. Burn it down, brother. Burn it all down!"
- —Jerome Valeska announcing his legacy in a recording for Jeremiah[src]
Revealing his dark side
- "'Jerome Victorious?' Are you serious? He's dead, haven't you been paying attention? I am the one who's victorious. Look, Bruce, like everything Jerome set his mind to, his insanity gas failed. Other than some... mild cosmetic effects, he might as well have sprayed me with water. You all needed to see Jerome for the utter dud that he was, so I donned the mask of madness to show you how feeble that is... compared to actual greatness. Behold... the face of true sanity. But, looks aren't everything. I have a compendium of Jerome's obsessions and goals, and... I will outdo every one of them. Jerome wanted to turn Gotham into a madhouse. But to truly build something, you must first tear down what is already there."
- —Jeremiah Valeska to Bruce Wayne and the Cult of Jerome[src]
Jeremiah created a recording tape of himself disguised as his deceased brother Jerome and stated that he would have a wake at the GCPD. He then set Ecco to invite Jerome fanatics that were celebrating at his grave.
When Bruce Wayne visited Jeremiah Valeska, he showed Bruce Wayne the prototype of the device he was working on. He also pulled out Jerome's diary which Arkham gave to him after his death. There he revealed that the book was disturbing and holding him back but Bruce morally supported him by saying to quit the book. Shortly after, Bruce received a phone call about Jerome's wake and Jeremiah became paranoid. Jeremiah confessed that he was sprayed the Insanity Gas as Jerome's last sick game and Bruce convinced Jeremiah to visit Jerome's grave to truly convince the cowering Jeremiah that he was truly dead. After visiting the grave and finding it empty, Jeremiah runs away inside the cemetery building and when Bruce catches up to him, it appears as Jeremiah is losing his mind, now that he views that Bruce is his dead brother.
After Jeremiah chases Bruce towards Jerome's Graveyard, Jerome's body is now placed next to the tombstone. Still not buying the fact that his brother is dead, he pulled out a knife from Jerome's corpse and fought Bruce Wayne. Jim Gordon investigates he bunker and a recording starts playing. when Ecco arrived and placed a gun at his head, the recording starts and it looked like Jerome had an extra tape regarding Jim Gordon. After an extensive fight with Ecco, it is revealed to Gordon that Jeremiah was faking the Jerome act as he peels off the fake damaged skin and rubs the makeup off with a cloth.
Meanwhile, after a thug proclaims proudly "Jerome's victorious" Jeremiah murders him in cold blood due to him believing that he was the victorious one and not his brother since he was dead. Now cold and monotone, Jeremiah wiped the blood of his victim off his face, along with the makeup disguising his altered features; he apologizes for the act, saying it was to show Jerome's followers what an utter dud his brother was.
Kicking Jerome's corpse into the grave, he then revealed his plans for Gotham as he pulls out the diary and states he is going to outdo his brother’s goals in a sane matter. Bruce disregards his plan, saying it's complete madness and to carry out Jerome's plans in a "sane" manner. Jerome replies with an entry from Jerome's diary: "Jerome wanted to slather you in honey and have you eaten alive by corpse beetles. Now that's mad. Me, if I wanted to kill you I'd just do it. I'll shoot you in the head. Simply and sanely". However, Jeremiah then said he didn't wish to kill Bruce, as he truly does see him as his best friend. He then reveals to Bruce and Gordon that the generators he invested his money in can double as highly powerful bombs. Back at Jeremiah's residence the bunker's generator starts to overload, but luckily Jim finds a secret buzzer underneath the table and unlocks the door and escapes the bunker before it explodes. Jeremiah and Bruce both presume Jim has died, and Bruce angrily vows that he will stop Jeremiah. Jeremiah knocks him out and drops him into the grave with Jerome's corpse.
Jeremiah and Ecco then head to Wayne Enterprise and kill two guards to obtain their keycards. they open the locked door with the key cards and it is revealed that there is a room full of the powerful generators Jeremiah created and Bruce invested in. He then orders Ecco to move them as he plans to destroy Gotham and "Rebuild it." 
Plan To Rebuild Gotham
Jeremiah gives Bullock six hours to evacuate the city and destroys the Gotham Clock Tower to show he is telling the truth. Shortly afterwards, his men are shown knocking out and capturing Alfred Pennyworth at Wayne Manor.
Later, when Bruce meets with Bullock, Jeremiah calls him and tells him the address Alfred is at and warns Bruce not to tell the police as Jeremiah knows where Bruce is. Allying with Scarecrow, Jeremiah later watches Bruce via CCTV as he is sprayed with Scarecrow's fear toxin and forced to witness Alfred being tortured in an attempt to drive him mad. Jeremiah remarks that he "envies" Bruce, and that he wants him to be able to embrace his true, darker nature after being forced to lose everything and everyone he loves.
As Jeremiah enters his safehouse, he finds The Penguin, Barbara Kean, Tabitha Galavan, and Butch Gilzean waiting for him with one of Jeremiah's henchmen, Jongleur, hostage; they have also located and taken the cord relay that allows him to detonate all his bombs at once. They demand an additional hour to evacuate the city and $50 million. Jeremiah, knowing they will double-cross him anyway, pretends to agree to their demands and calls the mayor, but then claims he was put on hold, pulls out a bazooka and blows up Jongleur and the cord relay. Furious at Penguin's assertion that he is "out of his mind", Jeremiah remarks that he has always had a backup plan and can simply detonate the bombs in sequence instead. He then tells them he has changed his mind about the deadline due to their interference, and will detonate the bombs as soon as he's a safe distance away. Jeremiah orders his followers to "kill these idiots" and escapes during the subsequent firefight. Arriving at another bunker where the Cult of Jerome are gathered, Jeremiah prepares to detonate the bombs, but it's suddenly revealed that Gordon is still alive, much to his fury. Jeremiah triggers the detonator for the first generator, but it doesn't explode as Bullock found and disabled it just in time. The Cult of Jerome gets angry at Jeremiah, calling him a liar and a fake; he reacts by sealing them inside the bunker and incinerating them, saying he anticipated their betrayal. All but one of Jeremiah's generator bombs are retrieved by the GCPD, while Bruce is able to rescue Alfred and overcome Scarecrow's fear gas with the help of Selina Kyle.
Later, hiding out alone at an abandoned theater, Jeremiah wonders aloud how he was defeated; he concludes Gordon must have found his maze blueprints and sabotaged the sequence by finding and disarming the first bomb (which is almost correct, except for Harvey Bullock being the one who disarmed the bomb). As Jeremiah decides he must start over, he is interrupted by the appearance of Ra's al Ghul, who compliments him on his tenacity and suggests they can cause the destruction of Gotham together. Jeremiah sarcastically replies that "recent events have convinced me of the benefits of working alone" and tries to shoot Ra's, who easily evades his bullets. Realizing that his attempts to kill the Master of the League of Shadows are futile, Jeremiah reluctantly hears him out. Ra's explains that Jeremiah's plan for a "new Gotham" can still come to be with the help of the League of Shadows, and both their plans are not just about Gotham, but about Bruce Wayne, which draws Jeremiah's interest.
Later at Wayne Manor, Bruce and Selina are talking about Jeremiah's obsession with Bruce. Bruce explains that Jeremiah was working on the premise that "one bad day" can drive someone insane. He wonders if the trauma of his own parents' deaths "made him a little insane", and if Jeremiah had sensed that and was trying to bring it to the surface. As Selina replies that Bruce proved him wrong, Jeremiah suddenly enters the room and holds a pistol to her abdomen, remarking that the day's not over yet. He shoots Selina in the stomach, knocking her backwards, then smiles and throws his gun away. As Bruce desperately tries to keep Selina alive, Alfred rushes in, knocks Jeremiah down and begins pummelling him repeatedly in the face. 
Aligned with Ra's al Ghul
- "You're right, you know; I did fail. I didn't yet understand the vision I was servicing, but... I do now. My eyes are open, as will yours be. When you've realized I'm not bluffing, bring me Bruce Wayne. And I'd like a coffee. Black... two sugars."
- —Jeremiah Valeska to Jim Gordon[src]
After being arrested, Jeremiah is held at the GCPD. Talking with Gordon, Jeremiah mockingly says he wishes to press assault charges against Alfred for punching him in the face, to which Gordon (with equal sarcasm) replies that he thinks Jeremiah looks just fine, ordering Harvey to prepare to transport him to Arkham. Jeremiah offers to give up the location of "other" bombs he claims to have planted around the city, in exchange for being allowed to speak alone with Bruce. A few moments later one of his generator bombs (planted by Ra's al Ghul) goes off and destroys City Hall, killing the mayor and seemingly confirming that Jeremiah is not bluffing. Gordon attempts to beat the locations of the other bombs out of Jeremiah, who barely reacts, merely grinning and humming "Ode to Joy". A major finally has Bruce Wayne meet with the restrained Jeremiah, attempting to get the location of the other bombs. Jeremiah reiterates that he believes Bruce is his kindred spirit, and taunts him about Selina's injuries. When pressed about the location of the "other bombs", Jeremiah reveals that the only other bomb was the one he used on City Hall.
Jeremiah finally reveals to Bruce he is working with Ra's al Ghul. The lights suddenly go out, during which several guards are killed and Bruce and Jeremiah both vanish; they have both been evacuated from the GCPD by the League of Shadows, which also steals all of Jeremiah's confiscated bombs and re-distributes them across Gotham's bridge system. The League then brings Jeremiah and Bruce to an abandoned building overlooking Gotham River, where Ra's is waiting for them. Ra's and Jeremiah tell Bruce they'll create a legacy by destroying Gotham, allowing Bruce to be "reborn" as the Dark Knight of Ra's visions. Jeremiah admits he is skeptical of prophecies and visions, but the idea of he and Bruce rising out of the fall of Gotham together appeals to him.Shortly after, Barbara Kean, wielding Ra's cursed knife, arrives, referring to Jeremiah as a "freak". At first they think Barbara came alone, however, Penguin then started screaming and shooting a machine gun at the League, while the whole alliance of Tabitha Galavan, Alfred Pennyworth and the Sisters of the League appeared behind him and began battling Ra's al Ghul's men. Ra's defends himself against Alfred and Barbara, asking her if she will never learn, but she finishes the sentence for him, telling him she gets that the knife has to be held by Bruce Wayne only. Jeremiah uses Bruce as a shield and shoots a member of the Sisters of the League, but Tabitha's whip ties his hands together and pulls him away from Bruce. Galavan then laces her whip around Jeremiah's throat, trapping him in stranglehold and forcing him to his knees in front of her. She tells him "this is for Selina", but before she can deliver a killing blow, Valeska manages to draw Tabitha's own knife and stab her in the leg. Valeska then grabs Galavan, throws her on the ground and grabs her throat with one hand, holding her knife near her eye with the other, smiling sadistically while telling her that it must be very disappointing for her to die like this in case she wanted to avenge Selina. However, Valeska is suddenly shot in his shoulder by the Penguin. Surprised that Cobblepot, of all people, saved her life, Tabitha looks at him in confusion; she doesn't not know yet that he has his own reasons for keeping her alive. After knocking out Palden, Bruce turns to fight Ra's, but both he, Barbara and Alfred are kept at bay by the Master of the League of Shadows. While Ra's is fighting Alfred, Barbara forces the knife into Bruce's hands before ramming it into Ra's stomach. Jeremiah, still lying on the ground with an injured shoulder, watched this with surprise. As Ra's body began to disintegrate, he looked at Barbara Kean, saying "Well done, Barbara".
Right then, the sound of an explosion shakes the whole city, drawing the attention of Barbara, Bruce and Ra's, as well as the horrified Oswald Cobblepot and Jeremiah Valeska, the latter one smiling savagely as one of Gotham's main bridges collapses into the Gotham River. Multiple other bridges follow the same fate, crashing into the water after their connections to the mainland give in to the detonation of Jeremiah's generator bombs. Shocked by this sight, Penguin, Barbara, Tabitha, Bruce and Alfred witness power failures affecting the fallen city. The dying Ra's grabs Bruce's shoulders, looking into his eyes and telling him that the choice is now his: he can either stay Bruce Wayne or accept his destiny and become Gotham's dark knight. With that, Ra's al Ghul's body disintegrates, leaving only the knife behind. Horrified, Bruce turns to face the destruction of his city. By this point, Jeremiah Valeska has already taken advantage of the confusion and escaped. The alliance against him had failed to prevent the city's downfall.
Shortly afterwards, Bruce is seen interrogating a small-time criminal about Jeremiah's whereabouts; he tells him to let Jeremiah know he is coming for him.
Though he appeared to be more stable then his brother, calling him after his transformation a “Short-sighted psychopath, he wanted to destroy things”, Jerome claimed that Jeremiah was as crazy as he was. He was shown to be deceptive, as he claimed that Jerome had always been trying to kill him, though he later admitted to Jerome that it didn't happen exactly as he had described it. He tried to deceive Jim Gordon and Harvey Bullock that Jerome could not find him, even though he had already captured Jerome. He was overconfident, bordering on arrogance, on being able to keep his brother trapped, something that led to Jeremiah escaping and confronting. He was also willing to let people die if it meant he could avoid Jerome, showing that he was obsessed with his own self-preservation.
After being exposed to Jerome's altered laughing toxin, Jeremiah initially appeared to have been driven completely insane, laughing uncontrollably as he was transformed by the toxin. However, according to Jeremiah, his brother’s gas did little more than alter him cosmetically. Whether he was fully insane before the gas or if he is simply attempting to downplay his brother's effect on him is still unknown; his prior behavior did hint at some repressed mental instability, which Jerome's gas may have helped bring to the surface. Gordon theorized that, by destroying Gotham in such a way that it would form a "maze", Jeremiah was turning the city into the same environment he had always felt safe in, indicating that Jeremiah's mental instability may have originated from his years of hiding from Jerome, alone and underground.
Jeremiah has still shown himself to be highly skilled at deceiving people before and after his transformation, as he was able to act perfectly as Jerome on video tapes of his "will" and deceive Bruce Wayne into thinking he was having a mental breakdown, only dropping the act to illustrate his new sinister personality. Jeremiah has also shown to be quite calm for the most part, carrying out ruthless actions without the hysterical laughter and manic behavior displayed by Jerome; his mental illness has not affected his brilliant intellect, and he does seem somewhat more in control of his actions. However, he is easily as ruthless and cruel as Jerome if not more so, indicated by his casual destruction of Gotham's clock tower just to prove a point, and his shooting Selina Kyle and torturing of Alfred Pennyworth just to "release" Bruce Wayne from everything he loves. While Jeremiah was always deceptive, this degree of ruthlessness was never shown before his encounter with the gas.
Unlike his highly impulsive brother, Jeremiah continued to plan carefully and methodically after his transformation. Whereas Jerome always felt the need to have an audience to whom he could display his schemes and sociopathic tendencies, Jeremiah did not care in the slightest about having an audience; he was always focused on his endgame and usually had a backup plan, making it much harder to anticipate and defeat him. Also unlike Jerome, Jeremiah had more of a long-term vision, describing himself as a "builder" and "artist" and wanting to create something out of his destruction of Gotham, whereas Jerome only wanted destruction for destruction's sake.
However, despite appearing calmer than Jerome, he does sometimes display the same explosive temper when his plans are interfered with, screaming at his followers to prevent them attacking him, which led to him burning them all to death. He also shows a partial denial about his insanity, as he claims he is not insane several times and becomes enraged when anyone (other than Bruce Wayne) says otherwise. Like Jerome, he is something of a masochist after his transformation, being mostly immune to pain and merely grinning when taking a beating from Alfred and later Jim Gordon.
Jeremiah has also shown two significant obsessions. The first is surpassing his brother in every way, including carrying out his various schemes to destroy Gotham but more efficiently and on a greater scale. Jerome was the one thing Jeremiah always feared the most, and after his exposure to Jerome's gas (which was derived from a variation of Scarecrow's fear toxin) his need to surpass his brother could be his way of overcoming that fear. The second obsession is Bruce Wayne, who Jeremiah views as a kindred spirit and "the brother Jerome never could be" and is the only one to whom he (inadvertently) admits his insanity. Believing that Bruce's parents' deaths had caused similar mental trauma, Jeremiah makes many attempts to bring that darker nature to the surface, just as Jerome had to him. However, unlike Jerome (who was similarly obsessed with Bruce Wayne) Jeremiah makes it clear he has no intention of killing Bruce. After allying with Ra's Al Ghul and learning of his plans to transform Bruce into a "Dark Knight", Jeremiah believes he and Bruce are destined to rise from the ashes of Gotham together.
Originally, Jeremiah was identical to his brother, the only exception being his slightly darker hair. He wore glasses and dressed usually in an orange suit and blue collared shirt, and occasionally, a blue overcoat.
After his exposure to the insanity gas, Jeremiah's skin turned extremely pale, even paler than his brother's. In addition, his lips became bright red and his eyes piercing green, though he initially hid these changes with makeup and colored contact lenses. As Jeremiah removes both his glasses and (presumably) his contact lenses after revealing himself to Bruce (since his eyes are shown to change from blue to green in the next shot, and subsequently remain green), it appears the gas may have inadvertently improved his vision to the point that he no longer needs glasses. However, he does briefly wear dark sunglasses when he confronts Harvey Bullock outside the GCPD.
He later dyes his hair a very dark green and wears a purple suit and orange tie, covered with a plaid trench coat and red leather gloves; he later trades this suit for a purple and black one with a dark green tie, though he retains the gloves. Occasionally he is also seen wearing a wide-brimmed black hat.
- High-level intellect: Jeremiah is shown to be highly intelligent. Since he was a child, he used to draw mazes, and when he grew up, after having the best grades at school, he built one in his house, which he built underneath the woods. Furthermore, he is extremely manipulative as he was able to keep anyone from discovering his insanity for years, even after it was brought to the forefront by Jerome's gas, and he was able to convince everyone that he was Jerome in the tapes he sent to Jerome's followers and the GCPD. Jeremiah also claims to be 'vastly more intelligent' than Jerome was. He is a brilliant engineer, designing advanced generators capable of powering Gotham that could double as powerful explosive devices.
- High pain tolerance/Indomitable will: Jeremiah has shown to have a high tolerance for pain, as he showed no signs of pain when Alfred was punching him in the face repeatedly. He later showed only slight signs of pain when shot by Oswald Cobblepot with a machine gun.
- Master tactician: Using his vast intelligence, Jeremiah also filled his secret hideout with security cameras in order to be ready for his twisted brother's arrival, whilst also utilizing the right tactics in order to trap and escape Jerome.
- Evasion: Jeremiah has shown to be skilled in evading people, as he was able to evade Jerome for years.
- Master engineer: After graduating school, Jeremiah used his construction skills of labyrinths in order to build a secretive hideout for himself in order to defend himself from Jerome, which took him six years of construction.
- Impersonation/Acting: He was able to impersonate his brother Jerome in multiple video recordings, making him a very talented actor, even capable of fooling the whole GCPD and the Cult of Jerome with his disguise.
- Leadership Skills: Before his transformation, Jeremiah was shown to be timid and hesitant around other people, probably due to his long period of social isolation. However, after his transformation by the gas, he became much more confident and charismatic, enabling him to quickly take leadership over the Cult of Jerome and make alliances with Ra's al Ghul and Scarecrow.
- Make-up artist: Jeremiah is shown to be a skilled make-up artist on many occasions. After his skin was bleached white, Jeremiah was able to use make-up to hide his transformation from the public for weeks. He was even capable of taking on the appearance of his brother Jerome by the use of prosthetic make-up for the scars.
- Voice alteration: Jeremiah was able to change his voice to make himself sound like someone else. It was shown that he can convincingly imitate the voice of his brother Jerome, a very difficult task, since Jerome got his striking voice because he got stabbed in the throat before his first death, disfiguring his vocal cords permanently after his resurrection.
- Skilled Marksman: Jeremiah has shown proficiency with guns and other firearms. He was able to shoot and kill one of Jerome's followers without looking at the man behind him using his hidden sleeve gun.
- Skilled Knife Wielder: Jeremiah is skilled at using a knife. This is seen one he attacked Bruce with a straight razor and when he used Tabatha’s karmbit knife against her and almost killed her with it.
- Hand to Hand Combatant: Jeremiah has shown some skill in hand to hand combat. This is seen when he managed to pin Tabitha to the ground after stabbing her with her own karambit knife, though he had also taken her by surprise at the time..
- Arrogance: Jeremiah believed that he had captured Jerome and could keep him imprisoned. When Jim and Bullock attempted to put him in protective custody, Jeremiah firmly believed he had defeated Jerome. Only for Jerome to outmaneuver him by leading his allies to Jeremiah's location, hypnotizing his bodyguard, and revealing that he memorized the labyrinths that Jeremiah designed as a child. Jeremiah also arrogantly believed that his bomb had killed Jim Gordon, even though he had no proof that he had perished.
- Jerome: Jeremiah's greatest weakness is Jerome, his own twin brother. Since a young age Jeremiah supposedly suffered from Jerome's twisted and sadistic personality this led to Jeremiah suffering at the hands of Jerome who tormented him violently and sadistically, at one point during this period Jerome even set fire to Jeremiah's bed while he was still in it. Jerome also wished to have Jeremiah carry on his legacy by spraying him with a “special gas” that could drive him insane. According to Jeremiah, the gas merely changed his face and did not have any effect on his true deranged mentality. However, it has yet to be determined which of the brother's claims are more accurate, as it was in Jeremiah's best interests to deny credit to his brother as much as possible as well as having previously lied about some of Jerome's actions.
- Guns/Firearms: He uses different firearms, primarily handguns, which he appears relatively proficient with. He used a small derringer pistol that retracts from his sleeve to kill a Jerome follower.
- Knives: Jeremiah has used different types of knives. He used a spring-assisted knife given to him by Jerome to try to kill him and make him given to his killer instincts. He used a straight razor taken from Jerome’s corpse to attack Bruce. He used Tabitha’s karambit knife against her and almost killed her with it.
- Bombs/explosives: Valeska used a wide variety of explosive devices, particularly the specialized generators he had designed and built with the help of Wayne Enterprises. He modified them to serve as bombs; just one was easily capable of leveling a skyscraper. Being a brilliant engineer, he designed highly sophisticated and efficient detonation systems for his explosives as well. Jeremiah also used a bazooka/ rocket-propelled grenade to destroy Jongleur during his confrontation with Penguin and the Sirens.
- Jack-in-the-box: Jeremiah kept the jack-in-the-box that sprayed him with Jerome's laughing gas and later used it to traumatize Bruce Wayne while trying to prove to him that one bad day is all it takes to reduce the sanest man alive to lunacy.
- "A Dark Knight: Mandatory Brunch Meeting"
- "A Dark Knight: That's Entertainment"
- "A Dark Knight: That Old Corpse"
- "A Dark Knight: One Bad Day"
- "A Dark Knight: No Man's Land"
- According to executive producer John Stephens, Jeremiah is not the Joker in Gotham and the real Joker is watching the actions of Jerome, Jeremiah, and possibly a third, adopting them as inspirations. This statement is considered the official canon until further notice.
- Although not confirmed, it is possible that the "someone else" is the Jerome doppelganger briefly seen and heard in "Wrath of the Villains: Transference".
- Cameron Monaghan stated in January 2019 that "the character I most enjoyed playing is the one in the final episode that we haven't seen yet." In the same interview, he revealed that the Joker is not a biological triplet of Jerome and Jeremiah, and that Jeremiah has a "linear arc" in Season 5.
- Monaghan stated that Jeremiah's final scene in Season 5 is "vile, vicious, nightmare-fuel horror and is everything I love about this character."
- The Joker made his first appearance in Batman #1 (April 25, 1940). He is credited as having been created by Jerry Robinson, Bob Kane, and Bill Finger.
- Jeremiah Valeska is heavily based on the original Joker from the Golden Age of Comic Books and the Joker from Frank Miller's 1986 Batman: The Dark Knight Returns storyline, where the character was portrayed as more cold and calculating than humourous.
- Like the original Golden Age Joker, Jeremiah Valeska's hair appears in a very dark green tone, almost seeming black in certain light conditions.
- Jeremiah's outfit with the purple hat and coat is an allusion to clothes the Joker wore in in the comics on multiple occasions, most famous in Batman: The Killing Joke, as well as the 1989 live-action movie Batman.
- When Jeremiah is undergoing his villainous transformation, he clutches his head in a way that mirrors the famous panel of The Joker going insane in Batman: The Killing Joke.
- Xander's assumed surname could be a reference to the wild and unpredictable nature of the Joker. It could specifically be a reference to an episode of Batman: The Animated Series called "Joker's Wilde".
- Jeremiah and Jerome share the same facial features apart from Jeremiah's hair being dark brown, with Jerome's being red, and Jeremiah wearing glasses.
- It is possible that Jeremiah's stories about Jerome being a psychopath as a child, are fictional and purely a ploy to get sympathy from those around him.
- Even though Jeremiah acts as Gotham's precursor to the Joker, he will never be called "The Joker" as Cameron Monoghan confirmed on his twitter account due to Warner Bros. wanting to use the name "Joker" for future projects. Cameron also confirmed Jeremiah cannot have bright green hair.
- This has led to a small Campaign called #LetGothamUseJoker; the reason likely being that Gotham is going into its final season, and afterwards the name "Joker" could be used only by Warner Bros.
- Jeremiah stated that being sprayed with Jerome's special laughing gas caused no change in his mental stability, seemingly indicating that he was insane before he was sprayed and the gas simply brought his insanity to the forefront of his mentality. It is unknown though how true this statement is, as Jeremiah may be simply in denial about the effects of his brother's actions on his current mental state.
- ↑ https://www.youtube.com/watch?v=iwd13B24cXY
- ↑ Lilien, Steven & Wynbrandt, Bryan (writer) & Vrvilo, Maja (director) (April 5, 2018) "A Dark Knight: Mandatory Brunch Meeting" Gotham. Season 4. Episode 17.
- ↑ Cannon, Danny (writer) & Copus, Nick (director) (April 12, 2018) "A Dark Knight: That's Entertainment" Gotham. Season 4. Episode 18.
- ↑ Huston, Charlie (writer) & Milto, Louis Shaw (director) (May 3, 2018) "A Dark Knight: That Old Corpse" Gotham. Season 4. Episode 20.
- ↑ Chun, Tze (writer) & Bailey, Rob (director) (May 10, 2018) "A Dark Knight: One Bad Day" Gotham. Season 4. Episode 21.
- ↑ Stephens, John & Boston, Seth (writer) & Hope, Nathan (director) (May 17, 2018) "A Dark Knight: No Man's Land" Gotham. Season 4. Episode 22.
- ↑ http://m.ign.com/articles/2018/05/10/gotham-producer-talks-joker-swap-no-mans-land-and-eventually-ending-the-show-with-batman
- ↑ https://ew.com/tv/2019/01/10/gotham-cameron-monaghan-final-season-arc/
- ↑ https://comicbook.com/dc/2018/11/09/gotham-cameron-monaghan-jeremiah-final-appearance/
- ↑ https://twitter.com/cameronmonaghan/status/995422386437570560