Joker (Justice League Action)

Joker is the secondary antagonist in Justice League Action. He is Gotham's most notorious criminal mastermind and Batman's archenemy.

He is voiced by Mark Hamill who has also voiced multiple incarnations of the Joker over the years.

Biography
Jack Napier was a lab technician who was down on his luck. After his failed career as a comedian, he was unable to support his pregnant wife Jeannie and had no choice but to turn to crime. Jeannie later died due to a malfunctioning water heater. Under the guise of the Red Hood, he and the Valestra mob planned to rob a chemical plant. Unfortunately, Batman attacked and Napier fell into a chemical vat, changing his appearance into that of a deranged clown. Driven insane by his new look, Napier called himself the Joker and has been terrorizing Gotham City ever since.

After becoming the Joker, he continues to cause mayhem at Gotham City, enjoyed torturing Batman as revenge on him for his disfigurement which he blames him for and was acquainted with Harley Quinn numerous times.