Their main strength is to attack the hero’s weakness: the more powerful the antagonist, the more difficult it is for the protagonist. Batman’s power comes from his ability to intimidate people, as well his physical strength, which is reflected in the sharp lines and dark colours of his costume. The Joker is able to create situations where he renders his powers useless, such as when he captured Rachel and Harvey Dent. Also, the fact that The Joker doesn’t fear death means that Batman’s power to intimidate cannot be used against him. Batman refuses to kill people, even criminals, and the only way to stop The Joker would be to kill him; this is the ultimate dilemma.
Forcing the protagonist to make difficult choices: the more difficult choices the protagonist has to make, the truer the choice is to reflect that character. The Joker pressures Batman into making difficult decisions throughout the film, which get more and more serious as more people keep dying. This is clearly seen where Batman has to choose between Harvey Dent and Rachel.
The Joker makes Batman wiser to his nature. At the beginning of the film, Batman expresses how he believes that all villains must have a desire for something, however by the end of the film he understands how The Joker only wishes to cause chaos and destruction.