Jack Napier

"I should be hitting the museums, snapping selfies, and stuffing my face with Earth-2 delicacies. But it’s hard to enjoy a trip when you’ve just witnessed the violent death of your evil doppelganger and saw your best friend kidnapped by a maniacal speedster who makes Heath Ledger's Joker look like Santa Claus."

- Cisco Ramon

The "Joker" is a criminal from Gotham City, and an enemy of Batman, responsible for the attack on Kane family. Due to his notorious criminal record, he was portrayed, presumably in pop culture, by the actor Heath Ledger and referenced by Cisco Ramon.

History
The Joker was a mysterious criminal, known only by his alias, and an enemy to the similarly half-mythical Gotham City protector Batman. In January 2003, he hijacked a bus full of schoolchildren and used it to attack the Kane family car, trying to ram it, so the family would fall from the bridge. He was stopped by Batman, who secured the car with his grappling hooks and pursued the Joker. However, the hooks turned out to not be enough, due to an engineering mishap in the car, causing it to fall into the river, leading to the death of Gabi Kane and the apparent death of Beth Kane.

Joker was later portrayed by Heath Ledger in a fictionalized form of some sort. Cisco Ramon mentioned him in his blog while discussing the events from his journey to Earth-2. Cisco compared Heath Ledger's Joker to Zoom, claiming that the maniacal meta-human speedster made Joker "look like Santa Claus".

Behind the scenes

 * In DC Comics, the Joker is the arch nemesis of Batman.
 * In, when Joe, Cisco and Barry rescued Harry, they find him being held by Griffin Grey at Ace Chemicals. In the comics, it's at Ace Chemicals that the DC Comics' Joker eventually fallen into a pit of chemical product that turned him into the maniac that he is.
 * The Joker, played by Heath Ledger, was indeed a character in The Dark Knight, a film adaptation of DC's Batman mythos. Considering the Joker's later appearance in Batwoman, it can be assumed that Ledger portrayed the character in a documentary, or as a fictionalized version of him in a movie based on real events (considering that Batman himself is thought to be a myth by a number of characters in-universe; Oliver Queen being one-such individual).