The Joker is the first appearance of Jack Napier in the universe of Howard Pacone. He is leading a gang, until Howard crosses his path. They might have been friends and worked together, had the Joker not decided to betray Howard in City of Broken Dreams. His decision led not only to Howard taking revenge, but also to him changing his path to crime-fighting and becoming the Shady Avenger, which means: Had it not been for the Joker, there wouldn't have been an Avenger at all.

The Joker in Los Santos
City of Broken Dreams
City of Broken Dreams 2 - Payback Time