The Joker

The Joker was a serial killer and a super-villain, a dangerous madman who dressed like a clown and committed violent crimes. He was often recognized as Batman's greatest enemy, living in and terrorizing Gotham City, although he spent most of his time in Arkham Asylum. In addition to being a deadly and unpredictable physical combatant, he was a brilliant and ruthless criminal mastermind. His crimes required no motivation other than his sadistic desire to show people the meaninglessness of life through pain and death, and the narcissism to see the world remade in his own image. Batman was his only worthy opponent, and although he passed up multiple opportunities to murder the dark knight, he had no such problems with killing Robin or crippling Batgirl. In addition to this haunting man's persona, it was discovered by Batman that there were more than one clown-prince of crime.

After the first inspired many copycats to occur, The Joker formed a sort of cult of only those he deemed worthy to enter. They merged and muddled their different histories so after a while they simply had no idea about their own origin therefore all becoming the joker that many fear.

The Original
The original and arguably the true joker is the oldest of them all but also the one surrounded in the most mystery. The original joker was around 100 years before Batman and was known as the pale man, many thought him to be a demon with pale skin, operating from the shadows and bringing with him different forms of Chaos and death.

In actual fact he was once a normal human, born to two parents like all other humans, he was just a man, who had murdered a few times in his life, and came across a Lazarus pit. After bathing in the pit the pale skinned man gained true immortality meaning he could live for ever.  When fighting Batman he was more of a gangster 

The Apprentice
A psychotic anarchist mastermind who portraying himself as an agent of chaos, who rose to power in the criminal underworld by thrusting Gotham City into turmoil and drawing Batman ever closer to crossing the fine line between heroism and vigilantism. Described as a "psychopathic, mass murdering, schizophrenic clown with zero empathy", he is potent in both mental combat and strategic planning, and his physique was defined by his clown makeup and gruesome Glasgow smile to add sick comedy to all his crimes and misdeeds.

Before becoming the joker he was an American man who joined the military to escape his troubled home life caused by abuse from his father when he was a child. He became a special ops soldier as he was very skilled with firearms, explosives and tactical planning but also was prone to outbreaks of anger and ruthless unjustified killings. Due to his behaviour he was served dishonourable discharge rather than further court action due to incredibly severe Post Traumatic Stress Disorder (PTSD), but was not sent home due to an attack on his camp. After the attack was repelled the camp was evacuated by one of the man's old squad mates shot him in the chest and left him for dead. The original joker must have been watching this man as he managed to save him by taking him to the Lazarus pit. He bathed the injured soldier for long enough that he would heal but not gain true immortality, however the soldier, after being taken out of the pit had gained practical immortality as well as healing but neither of them knew this at the time.

The New
The most psychotic of the jokers. He was an engineer who quit his job at a chemical company to become a stand-up comedian, only to fail miserably. Desperate to support his pregnant wife, Jeannie, he agrees to guide two criminals into the plant for a robbery. During the planning, the police inform him that his wife has died in a household accident involving an electric baby bottle heater. Grief-stricken, the engineer tries to withdraw from the plan, but the criminals strong-arm him into keeping his commitment to them.

At the plant, the criminals make him don a special mask to become the infamous Red Hood. Unknown to the engineer, this disguise is simply the criminals' scheme to implicate any accomplice as the mastermind to divert attention from themselves. Once inside, they almost immediately blunder into security personnel, and a violent shootout and a chase ensues. The criminals are gunned down and the engineer finds himself confronted by Batman, who is investigating the disturbance.

Panicked, the engineer deliberately jumps into the chemical plant's chemical waste catch-basin vat to escape Batman and is swept through a pipe leading to the outside. Once outside, he discovers, to his horror, that the chemicals have permanently bleached his skin chalk white, stained his lips ruby red and dyed his hair bright green. This turn of events, compounding the man's misfortunes of that one day, drives him completely insane, causing him to break out into hysterical laughter.

After his mental health deteriorated he actively looked for the one they called the joker, yet he never found the man he seeked. Disappointed and confused, the man decided the joker must have perished or have simply been a myth so he decided to take on the mantle and caused mayhem for batman for years.

One day, the Joker kidnapped Gordon, shooting and paralyzing his daughter Barbara, and imprisoned him in a run-down amusement park. His henchmen then stripped Gordon naked and caged him in the park's freak show. He chained Gordon to one of the park's rides and cruelly forced him to view giant pictures of his wounded daughter in various states of undress. Once Gordon completed the maddening gauntlet, the Joker ridiculed him as an example of "the average man", a naïve weakling doomed to insanity.

Batman arrived to save Gordon, and the Joker retreated into the funhouse. Batman entered the funhouse and faced the Joker's traps while the Joker tried to persuaded his old foe that the world is inherently insane and thus not worth fighting for, also claiming that Batman must have suffered from a bad day himself and gone insane as well, mostly because of his attire. Eventually, after a confrontation Batman tracked down the Joker and, after explaining that the reason he isn't laughing at all, nor seeing it as a funny side is because he's heard the joke before, and it wasn't funny the first time around, subdued him. Batman then attempted to reach out to him to give up crime and put a stop to their years-long war; otherwise, the two would be eternally locked on a course that will one day result in a fight to the death between them. The Joker declined, however, ruefully saying "It's too late for that...far too late." He then told Batman a joke
 * "See, there were these two guys in a lunatic asylum... And one night, one night they decide they don't like living in an asylum any more. They decide they're going to escape! So, like, they get up onto the roof, and there, just across this narrow gap, they see the rooftops of the town, stretching away in the moon light... stretching away to freedom. Now, the first guy, he jumps right across with no problem. But his friend, his friend did not dare make the leap. Y'see... Y'see, he's afraid of falling. So then, the first guy has an idea... He says 'Hey! I have my flashlight with me! I'll shine it across the gap between the buildings. You can walk along the beam and join me!' B-but the second guy just shakes his head. He suh-says... He says 'Wh-what do you think I am? Crazy? You'd turn it off when I was half way across!'"
 * ―Joker's joke

The joke was funny enough to make the normally stone-faced Batman laugh but then Batman pulled out a knife and stabbed it through the heart of the Joker leaving him there to die. It was while he lay their bleeding that the other jokers collected his body and saved his life, allowing him to join their Trio.

The Missing Joker
The was at one point another joker. Before the newest joker joined there was actually another who was part of the group but he vanished and little is known about him. It was said that he blew up an entire city in an attempt to fall society, and after this the other Jokers sought him out for his creative chaos.