The themes in 'batman vs joker graphic novel' are complex. There is the theme of morality. Batman adheres to a strict moral code, while the Joker has no such constraints. This creates a contrast that drives the story. The theme of the power of insanity is also present. The Joker's insanity gives him a certain kind of power, and Batman has to find a way to counter it without losing his own sanity. Additionally, the theme of the city as a battleground is important. Gotham is where their fight takes place, and it represents the struggle between chaos and order on a larger scale.
The main characters are, of course, Batman and the Joker. Batman is the Dark Knight, a vigilante who uses his skills and gadgets to fight crime in Gotham. The Joker is his arch - nemesis, a deranged criminal with a warped sense of humor.