Mad Love introduces Harley Quinn as a former psychologist named Harleen Quinzel who fell in love with the Joker during her internship at Gotham City's Arkham Asylum and details her transformation into the Joker's villainous accomplice Harley Quinn, as well as the Joker's lack of respect for Harley.

It is then revealed that the Joker's real name is "Jack Oswald White." Martha then shares that she visited the Flashpoint universe's version of Jack Oswald White, discovering him to be a struggling family man supporting his wife and a child as a janitor at Wayne Casino.

The Joker possesses no superhuman abilities, instead using his expertise in chemical engineering to develop poisonous or lethal concoctions and thematic weaponry, including razor-tipped playing cards, deadly joy buzzers, and acid-spraying lapel flowers.

the Joker, comic-book character and archnemesis of DC Comics' superhero Batman. The Joker is noted for his clownlike appearance and sick humour. The Joker, initially portrayed as a small-time crook, was disfigured and driven insane by an accident with toxic chemicals.