I also think that the Joker’s real name is unknown. The Joker has been established as an identity adopted by the most recent person who has dressed up in that persona, which they had originally inherited from Dr Hugo Strange. Aside from some limited flashbacks to when he was a child, there are no notable scenes of him before he became the Joker. In Batman #663 it’s told how this future Jokerette became fascinated with clowns and created her own alter-ego at age seven. He was born one year before Bruce Wayne, and five years after Batman first started wearing his batman suit. His parents were killed when he was young; however, according to Tim Drake his parents beat him mercilessly for what seemed like hours