Jack Napier, born 06-18-1967 in Gotham City, is Howard Pacones constant nemesis.
The background of Jack Napier is not well known. When he first appeared in Los Santos, he did it as the Joker, but he is not and never was the “real” Joker from Gotham City. But it is possible, that he was one of the original Jokers goons there.
On the other hand, in The Shady Avenger it is mentioned that he is a former employee of Humane Labs, who snapped after he saw what experiments were done there, and thus became the Masked Marauder.
Maybe he was both at the same time - a respected scientist at Humane Labs, masquerading as the Joker while commiting crimes. But that wouldn't explain why he definitively had issues with the nature of the experiments at Humane Labs, except if he still has some morale in him. As a character in the universe of Howard Pacone, he's a rather dark and enigmatic one.
As Howard took his revenge on the Joker in City of Broken Dreams 2 - Payback Time, the criminal empire of Jack crumbled to pieces, so he abandoned the character, also because of his intention to destroy Humane Labs. After he escaped that car bomb Howard had placed, he figured that's a good way to get rid of the Joker and start a new life (of crime, of course). So it is not impossible that he let Howard find him intentionally.
After surviving being shot down with a helicopter in The Shady Avenger, he underwent a plastic surgery to restore his face, and started Napier Incorporated together with his niece, Diana Napier from Liberty City.