Obviously, Dooku himself is a crucial character. He is the central figure around which the story revolves. Count Dooku is a complex character who transitions from a Jedi to a Sith. Another important character is Yoda. His relationship with Dooku, as his former master, adds depth to the story.
Well, in the novel, there are some other characters that are important too. For example, there are Jedi Council members. Their interactions with Dooku, their suspicions of him as he starts to stray from the Jedi path, are important plot elements. There are also some minor characters like Dooku's servants or allies on certain planets. They may not be as well - known as the others, but they contribute to the overall atmosphere and plot development of the story.