The exact origin of the Joker is unknown, and urban legends abound. According to some she's a manifestation of all the souls of those who died during the Cuban Missile Crisis (on Earth Tangent the missile were lauched), others say that she's a wacky space alien trapped on Earth.
Apparently only her ally/enemy officier John Kell of the New Atlantis Police Department came close to the truth. Mary Marvel was born about 10 years after the Crisis, one of the few children that didn't suffered from any kind of malformation from the radiation. She was considered a reason to smile in a grim world. Growing up she was acutely aware of the hypocrisy of the society she lived in, especially of the city of New Atlantis .
As a student at New Atlantis University she came into contact with the journalist Lori Lemaris and the reclusive millionare Christy Xanadu. They banded together and created the heroic identity of the Joker, so that people wouldn't forget the hypocrisy of the world.
The Joker was created by Dan Jurgens, Karl Kesel and Matt Haley as part of the Tangent line.
The Joker managed to obtain a great fame very quickly after her first apperences, her flamboyant style and her anarchic behaviour won her the praise of the younger generation and the hatred of the police. It was in under these circumstances that she met officier John Kell. Kell was the son of the soldier who personally pushed the lauch button during the Cuban Missile Crisis. The Joker tested him considering a joke itself that Kell was protecting the city built over the ashes of his father's actions. Actually Kell managed to reverse the situation and was able to uncover some truths behind the mask of the Queen of Chaos. They discovered that their goals were very similar: both were in New Atlantis so that people wouldn't forget the sins of the past.
Later the Joker joined the Secret Six, a secret superhero team. As part of the Six, Mary fought against Superman in Washtington DC. The Six lost the fight and she was imprisoned. Superman psychically interrogated her and then executed her, she was the first victim of his totalitarian policy against superheroes. Lori was incarcerated shortly after, while the whereablouts of Christy remained a mystery.
Lori was later freed during the revolution started by Lia Nelson after her return from Earth-1. She was offered back her old costume, but she refused still mourning the loss of Mary, instead she used the identity of another deceased hero: the Manhunter.
The main weapon of the Joker is the Punchline, a billy club topped with a cartoonish hand that can be used in melee, thrown and contains a grappling hook. She also uses other kind of "comedic weapons" similar to those of her Earth-1 omonimous, but less lethal. These are usually recovered from vending machines of Impulse soda, these machines are all over New atlantis and they're actually fakes.