The Joker (Batman: The Killing Joke)

Early life
Supposedly, the man who would become the Joker had initially been an unknown engineer at the Ace Chemical Processing Plant, as well as a married man and an expecting father who quit his job, both due to being creeped out by the place, and to pursue his dream job of being a comedian in an attempt to help people laugh and feel good. However, his job proved to be challenging at best, often times flubbing a punchline or freezing up on stage, and he and his pregnant wife, Jeannie were forced to settle in a cheap apartment complex where the landlady hated him. The man one night proceeded to return from his time at a Comedy Club, where it was implied that he had done a terrible job (he mentioned that the club indicated they'll call him back in, but he fears he may have blown a punchline). After Jeannie gave an "oh", he then proceeded to lash out at his wife due to frustration at his failing his job, only to profusely apologize to her upon realizing what he was doing via his behavior. He then vowed to find a way to get them into a nice neighborhood, with his wife assuring him that she'll love him the way he is, as he was "good in the sack" and makes her laugh.

Some time later, he eventually becomes desperate enough to provide for his family by trying to ask two shady men for some help, offering to aid them in terms of passing through the ACE chemical plant where he formerly worked at. The men agreed, and to ensure his anonymity, they also gave him a disguise for him to wear for the caper against the card company next door: the infamous Red Hood, whom the man recognized from the papers as being the same guy who raided the Ice company the previous month. Although he was reluctant to go through the plan, he nonetheless agreed to do so, for the sake of his family.

The weekend they were to do the heist, he then met with the group, also lying to Jeannie that he's going to a gala to ensure she doesn't get concerned about him. Before they could start the caper, however, two policemen arrive and bring him outside for a talk. He then learns from them that Jeannie had died from a baby bottle heater accident where she and her baby were electrocuted. Feeling there was no more point to go through with the heist, he attempts to call it off. However, the crooks strongarm him into participating in the heist anyways by strongly implying that they will kill him if he attempts to back out, thus leaving him with no other options but to do so.

The next night, the man, after reminiscing his walking to work, proceeded to dress up as the Red Hood, and when noted he has a funny-shaped head, mentions he probably should have been a clown instead of a comedian. However, the already bad night got worse when he and the crooks discovered that, since he left the company, they had installed security. Due to his red hood apparatus fogging up, he eventually couldn't see the ground below, and was nearly executed by the gang members until they were gunned down in turn, although not before they pinned the blame on him. The man then tried to flee, and then pondered out loud what he did to deserve what came to him, only to encounter a black demon-like creature advancing on him and indicating they've encountered each other before. Scared of the demon sent after him, he then tried to beg for him not to come any closer, but then tripped and fell into a drainage vat. After emerging from the drainage vat outside, the man staggered, and then removed his red hood, also confused and agitated as to why he's burning and itchy all over. However, he stops himself when he sees his reflection and found out why: his face, and most likely his entire body, due to being exposed to the chemicals, had been disfigured, giving him emerald-green hair, chalk-white skin, and ruby-red lips, giving him the appearance of a clown. This, alongside the awful events of the entire night, drove him completely insane by having him laugh hysterically in anguish at the discovery, thus giving birth to the Joker. Whether this was actually true or not is unknown, as Joker later admitted that even he isn't sure exactly what transpired on his day since his memories tended to change every time.

He then grew to become one of Gotham City's most infamous criminals and Batman's most well known villain, where he had frequent bouts of being locked up in Arkham Asylum. Notable crimes of his included the murder of Robin. He also whenever getting out of Arkham would as his first act hire a prostitute to sleep with. He also proceeded to kill and lock up an audience, the remains of which would be uncovered three years later.

Main events
He then spent time fighting Batman various times, and being locked up in Arkham Asylum, with his frequent escape attempts having him see prostitutes. Some time later, he ended up trapping people and gassing them with Joker venom.

Three years later, his earlier action attracted the investigation of Batman after they uncovered the gruesome crime. Batman, who was planning on seeing the Joker anyways due to being troubled by their bleak future of ending their feud by killing one or the other and hoping to avert such a future, then proceeded to visit Arkham Asylum. Unfortunately, by the time they had arrived, Joker had not only escaped from the Asylum, but even went as far as to leave behind a body double in his cell, which Batman realized when talking with "him" due to his white "skin" coming off. He then went over to an abandoned amusement park, and after conning the carny into giving him control, poisoned him with Joker venom after gaining control of the park. He then arrived at Commissioner Gordon's place, proceeding to shoot Barbara Gordon in the gut in a way to ensure paralysis, and made sure to do it right in front of her father. He then forcibly abducted Commissioner Gordon. He then proceeded to sexually violate her and take pictures of her in various states of undress, although not before telling Barbara that he's doing his actions specifically to prove a point to her father.

Afterwards, he then had his goons strip Gordon naked and, after giving a philosophical speech about how Gordon is to be an experiment in regards to how even the most common man can be driven insane, then proceeds to psychologically torture the commissioner with a song about going insane and showing a slideshow of Barbara Gordon in various states of undress with the implication that he raped her. Afterwards, he then caged him up and showed him off as an exhibit just as Batman arrived. He then briefly fought Batman before fleeing, dousing him with acid. Confident in his victory, Joker then proceeded to tell Batman that even if he arrests Joker, he's already too late as Gordon's now insane (or so he believes), and proceeded to deduce that Batman had a bad day just like he did. He then ambushed him and gave him a beatdown in the upside down room of the funhouse, and proceeded to rant about how monstrous and pointless the world and morality is before demanding to know why Batman not only doesn't see the funny side, but isn't even laughing. Batman then turned the tables on his nemesis, literally, by flipping him right on the upside down table, before telling him that it's because he had heard the joke before and it wasn't funny the first time. Batman then told Joker that Gordon had told Batman to bring him in by the book, making clear that despite Joker's actions, he actually failed to drive Gordon insane, with Batman then theorizing that maybe not everyone cracked and maybe it was just Joker. Joker widened his eyes briefly at this possibility, but then denied it before trying to shoot Batman. Batman then tackled him out of the funhouse. Joker eventually retrieved his gun, but it ended up being a flag gun. Joker, frustrated by the failures, and possibly affected by the revelation that maybe he had been insane all along, then told Batman to just beat him up and give a standing ovation. Batman, however, refused, citing that if what Joker said about his having a bad day being what drove him to insanity was indeed true, he can help him recover from his insanity. Although Joker seriously considered the possibility of redemption, he sadly declined, realizing it was far too late for him. He then chuckled due to it resembling a joke he heard about inmates in an asylum. The joke ended up being funny enough that Batman, normally stone-faced, couldn't help but share laughter with his nemesis.

Trivia

 * Mark Hamill, Joker's voice actor, had previously played various other incarnations of the Joker, most notably the version from the DC Animated Universe, the version from Batman: Arkham Asylum, Batman: Arkham City, and Batman: Arkham Knight, and the version used for Birds of Prey. In fact, Hamill even offered to play this version of the Joker for free when an adaptation for Batman: The Killing Joke was made.
 * During most of the flashback portions, the man who would become the Joker sounded relatively normal sounding, yet after his trip through the drainage duct, he possessed the Joker's trademark high-pitched voice in the brief moment of sanity he had before seeing his disfigurement. This implied that his exposure to the chemicals not only altered his physical appearance, but also his vocal cords.