The crossover event Elseworlds revealed that a surprising number of Batman characters exist in the Arrowverse. While previous episodes of Arrow, The Flash, and Supergirl have made off-hand references to The Dark Knight and had some of his enemies make an appearance, for the most part, the Arrowverse hasn't explored the world of Gotham City until now.
Elseworlds explained the reasons for this somewhat, with Oliver Queen believing - like most of the world, apparently - that Batman is an urban myth created by the Gotham City Police Department to scare criminals into behaving themselves. Barry Allen, by contrast, fully believed in Batman's existence. This all seemed rather ridiculous to Kara Zor-El, who knew for a fact that Batman is real on her home-world of Earth-38 and is somewhat friends with her cousin, Superman.
What follows is a quick rundown on every character from the Batman comics whose existence in the Arrowverse was confirmed during Elseworlds. While not every character appeared on camera, the reality of them being in the Arrowverse was established either by context clues in the background, name drops by other characters, or more than a few clever Easter eggs.
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Upon first arriving in Gotham City, Oliver suggests looking up Vesper Fairchild - a radio personality who knows everyone in town worth knowing. Oliver reveals that he met her several years earlier and that they may have had a one-night stand together, but he couldn't be certain as that was during his drunken playboy days. (It was later confirmed that they had, after she hung up on him.) In the Batman comics, Vesper Fairchild was a radio talk show host, the self-dubbed "Siren Of The Night," who dated Bruce Wayne exclusively for a time until she was killed as part of a plot to frame him for her murder.
Kate Kane is introduced to the Arrowverse heroes after she pays their bail when they are arrested shortly after arriving in Gotham. Revealed to be Bruce Wayne's cousin, who is in the midst of trying to renovate the former Wayne Enterprises building, she assists them only because she hopes it will get infamous vigilante Oliver Queen out of her city faster. She later orders them out of her city as Batwoman when their efforts to confront Dr. John Deegan start a full-scale riot at Arkham Asylum. Despite this, she does seem to built a rapport with Supergirl, who is the only person that realizes Kate Kane is Batwoman after she uses her X-Ray vision to peek through Batwoman's mask. It remains to be seen if The CW will move forward with a Batwoman TV show or not.
The Arrowverse's "Elseworlds - Part 2" revealed that Bruce Wayne disappeared from Gotham City some three years earlier, shortly after the last reported sighting of Batman. Nobody thought much of this, however, as many people left Gotham City after the rumors of Batman's disappearance, as Gotham City somehow found a way to become even more crime-ridden and generally awful. While it is never outright said that Bruce Wayne and Batman are the same person, Kate Kane doesn't correct Supergirl when she tells her in private that she thinks her cousin would be proud of what she's doing after they stop a riot at Arkham Asylum.
Bruce Wayne's butler and foster father, Alfred Pennyworth, doesn't make an appearance in Elseworlds. Indeed, there's no indication one way or the other that he's still in Gotham City or that he disappeared as mysteriously as Bruce Wayne. Still, his existence is acknowledged in a rather humorous way: After telling Barry, Kara, and Oliver that they can use the Wayne Enterprises building as a base of operations, Kate Kane informs them that the WiFi password for the building is "Alfred."
Oswald Cobblepot / The Penguin
As Oliver Queen and John Diggle go in search of Dr. John Deegan's office in Arkham Asylum, they wander down a hallway past several cell doors. The first door they encounter says the cell belongs to O. Cobblepot - a nod to Oswald Cobblepot, aka The Penguin. This is rather unusual, as The Penguin is not considered an insane criminal in the comics, having made his nest at Blackgate Penitentiary on those few occasions with Batman caught the foul fiend in the act.
Dr. Pamela Isley / Poison Ivy
The next cell down the hall from The Penguin belongs to P. Isley, which is clearly a reference to Dr. Pamela Isley aka Poison Ivy. If the name were not a dead giveaway, someone appears to have painted a red rose on Ivy's cell door. Perhaps the decoration was a gift from Harley Quinn the last time she was visiting Arkham?
Basil Karlov / Clayface
Room 303 has a rather dirty door and is labeled "Karlo, B." While not quite as famous as The Penguin or Poison Ivy, most DC Comics fans will recognize this as a reference to Basil Karlo aka Clayface. While the character's origins have changed dramatically over the years and there have been several villains to use the name, the current Clayface in the comics was an actor who had his body chemistry changed by a discontinued make-up called Re-Nu, thus transforming him into a shape-shifting mass of living clay.