The Joker appeared in the 1940 comic book called Batman. Joker's appearance usually varies depending on different artists and media, but typically he wears a lavender suit with matching trousers, gloves, a necktie and a flower on his chest that shoots acid. Of course, aside from his clothes, Joker has chalk white skin, green hair, green eyes and a big ghoulish smile on his face with an unusually long nose and chin. On such occasion, Joker is also seen wearing a wide brim hat.
Joker has been known to wear different outfits depending on his mood. In the infamous graphic novel The Killing Joke he wore an outfit to resemble a tourist on a tropical vacation.
In the Death of the Family comic arc, Joker was determined to “fix” what he saw wrong with Batman and wore an outfit to show this. He wore a blue mechanics jumpsuit with the words “Joe’s Garage” labeled on the back, a tool belt around his waist and, most noticeably, had the skin of his face hastily strapped back on his head with a belt and twine after cutting it off a year prior.
In the comic arc Endgame, Joker returned with a new appearance. His face had completely grown back with a few alterations due to the healing properties of the chemical Dionesium. The length of his nose and chin are less exaggerated then they were before and his iconic grin, while still very wide, is not as long as it use to be. Joker also sports a new hairdo and wears a black suit ensemble with a pair of white gloves. He eventually wears another outfit more in line with his classic attire, although changes to his hair and facial features remain.
In the mini-series Underworld Unleashed, The Trickster remarks, "When supervillains want to scare each other, they tell Joker stories". As far as truth about Joker’s personality, one word sums him up, unpredictable. One day, the Joker may be a harmless clown, to others he can become a crazy, maniacal, lunatic, psychotic serial-killer.
Various psychologists in Arkham Asylum have tried to diagnose, though unsuccessfully, the nature of the Joker's mania. It has been stated that he tests positive for all of the clinical markers for both sociopathy and psychopathy although given his unpredictability and volatile nature, the Joker would be closer to a psychopath. It is believed that a combination of trauma of falling into the chemicals that changed him and the chemicals affect on his system that twisted the Joker into what he is which is partially supported by the fact that the Lazarus pit temporarily cured him of insanity and the pit is known for its healing properties.
At his most harmless, the Joker is still menacing and will endanger lives with no regard for the harm he causes. At his worst, the Joker would be best described as a living nightmare. A creature whose only purpose is to bring pain and death for his own perverse amusement.
In addition to this, the Joker is one of the few villains attributed with killing one of Batman's sidekicks (the other being Black Mask). The Joker beat Robin/Jason Todd to death in the comic "A Death in the Family". For any Batman villain, killing a Robin is almost the peak of villainy. Only two Robin’s and one Batgirl have ever been maimed or killed at the hands of supervillains.
He also indicates that all it takes is one bad day for even the nicest most upstanding of people to go insane, citing his own experiences, as well as claims that life is all "one big joke" and that the only sensible way to live is either by being insane or by living without rules, strongly implying that he is a nihilist.
However, even the Joker has his standards. During a DC/Marvel crossover, while Joker is working with the Red Skull, he thinks his partner in crime's trademark swastika is merely part of a costume. However, when the Red Skull revealed himself as an actual Nazi, the Joker is horrified and in the end, he fights him as he is about to drop a bomb on Washington DC. In addition, he is contemptuous of white-collar corporate crime, as evidenced by his hatred of Warren White, a corrupt CEO who embezzled his client's money; he says that, while he may have killed people, he "didn't steal their kids' college funds." Another noticeable moment is at the end of the "No Man's Land" story arc, in which the Joker kidnapped several infants and held them hostage. Sarah Essen Gordon tried to rescue them, but he disarmed and killed her during her attempt. He found no humor in her death, frowning as he walked away and promptly surrendering to the police.