The Joker is a psychopathic and homicidal maniac based in Gotham City, and is by far the most dangerous adversary that Batman has ever faced. Joker has a colorful personality, and tends to use theatricality and clowned or circus themes in his appearance and equipment. Completely unpredictable, with no regard for human life, the Joker revels in every moment of chaos he creates, and is often joined by his accomplice and girlfriend Harley Quinn, whom he had once turned psychotic. No one really knows who he was before and where he came from...
Personality
The Joker is characterized as nihilistic, maniacal, sociopathic, sadistic, homicidal, narcissistic, and egotistical. He completely unpredictable and utterly malevolent, with no regard for human life, and takes sadistic pleasure in the murder, mayhem and physical, mental and emotional suffering of others, easily managing to find humor, or even sport, in it. The reasoning behind his plans and crimes, as well as the mayhem and suffering he causes, usually only makes sense to him. Despite his diabolical and maniacal personality, he is shown to actually be very intelligent and in his schemes, the Joker usually manages to stay one step ahead of the police, and sometimes even Batman himself. He is a genius at planning and improvising criminal activities, despite his personality being capable of sometimes impairing his higher thinking.
The Joker possesses an obliviousness (or carelessness) to danger that makes him invulnerable to fear, which is usually Batman's greatest weapon in combat, making the Joker one of the Caped Crusader's most formidable enemies. His capacity for heinous torture and murder are enough to frighten low-level thugs and make them work for him out of the sheer fear of what he would do to them if they do not. Hence, the one quality that Joker has in common with Batman is the very intimidating presence that he commands, which gives Joker a tough and bold attitude. The Joker does not fear death and will even threaten his own life to achieve his goal or make a point.
