Jack Napier

"I should be hitting the museums, snapping selfies, and stuffing my face with Earth-Two 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 Clause."

- Cisco Ramon

Joker was a fictional character. The character was portrayed by Heath Ledger.

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

The Chronicles of Cisco

 * Post 27 (February 16, 2016)

Behind the scenes

 * In DC Comics, the Joker is the arch-nemesis of Batman.
 * The Joker, played by Heath Ledger, was indeed a character in "The Dark Knight", a film adaptation of DC's Batman mythos. It is unclear whether the fact that the Joker was mentioned by Cisco Ramon in his in-universe blog means that Batman and Joker do not exist in the universe shared by "The Flash" series, or that Heath Ledger's Joker is supposed to be some other kind of character, not related to the DC one.