Jack Napier is an alter-ego associated with the Joker, sometimes used as the character's real name. This is a tribute to the actors Jack Nicholson and Alan Napier. It has been used by several incarnations of the Joker, although usually as an alias and not a secret identity.
The "Jack Napier" identity was created by Sam Hamm and Warren Skaaren for the Batman film (1989).
Jack Napier was the narcissistic right-hand man of Boss Carl Grissom. Napier was having an affair with Grissom's girlfriend, Alicia Hunt, prompting the jealous crime lord to set his right-hand man up to be killed by the corrupt police officer Lt. Eckhardt at Axis Chemicals. However, Grissom's plan went awry thanks to intervention by both Batman and Commissioner James Gordon. After killing Eckhardt and then catching a ricocheted bullet in the face, Napier tumbled into a vat of chemicals.
Although Napier survives, the chemicals wreak a horrible toll: his hair is dyed green, his skin is bleached chalk white, and a botched attempt at reconstructive surgery severs critical facial nerves, leaving him with an eternal "smile".
After his mind completely snapped after seeing his smile, he assumes his new identity as the Joker, he kills Grissom as revenge ("You set me up over a woman".) and takes over the gangster's empire, engaging in a violent, chaotic crime spree, the motive being to "outdo" Batman, who he feels was getting too much press. He also tries to woo Gotham Globe reporter Vicki Vale.
When Bruce Wayne learns about the Joker, he recalls that his parents were murdered by Jack Napier, realizing that the Joker is indirectly responsible for the origin of Batman.