The Joker

A unnamed man known as The Joker is a psychopathic supervillain who's known to be the most notorious criminal in Gotham. He is the archenemy of Batman.

Biography
At some point, The Joker was the cause of an incident in which a lot of people died, and while Batman and Robin tried to stop him, he managed to get away.

Season 1

 * "Titans" (mentioned)
 * "Donna Troy" (mentioned)
 * "Dick Grayson" (fake reality)

Behind the scenes
In the DC comics, The Joker is psychopathic super villain and the arch enemy of Batman.