The main characters are Batman, of course. Also, the Joker is a major character. Robin is there as well. And there are some police officers who play important roles in the story as they interact with Batman, either trying to stop him or reluctantly working with him.
Well, in 'the dark knight returns story', the story is set in a future Gotham. Batman has been retired for a while. But crime is rampant, and he decides to come back. He starts to fight against the crime lords and also has to handle the return of his arch - nemesis, the Joker. And there are also some political and social elements as the city and the authorities react to Batman's return.
One of the main themes is the idea of an aging hero. Batman is older but still feels the need to fight for justice in Gotham. Another theme is the corruption and decay of society. The city has gone downhill, and Batman emerges from retirement to try and combat it.
In addition to those, there are other characters like Carrie Kelley who becomes the new Robin in the novel. She adds a new dynamic to Batman's world. There's also Superman, who has a complex relationship with Batman in this story. Superman represents the establishment and the government's power, while Batman is more of a vigilante figure. Their interactions are crucial to the plot.
Yes, Batman did kill in The Dark Knight Returns comics in some instances, but it was within a specific context and for particular reasons related to the storyline.
Well, there is a strong theme of redemption as well. Batman is coming back after years of being away, trying to make up for the state Gotham has fallen into. And the theme of resistance against a corrupt system is also there, as he battles not just criminals but also a system that has let the city decay.
Batman is obviously the most important. He's the central figure driving the story. Then there's Robin, who plays an important role in Batman's new fight for justice.