Rudolf Rassendyll, the story's protagonist, is by far the most important character. Rudolf is a cheerful young Englishman from an aristocratic family. As a wealthy young man from a privileged background, Rudolf leads a care-free existence, coasting through life without taking it the least bit seriously. As with many of his class and age, Rudolf has a real taste for adventure, and he gets the opportunity to indulge that taste to the full when he arrives in the kingdom of Ruritania.
The dashingly handsome young Englishman bears a staggering resemblance to King Rudolf V, who's been imprisoned by his younger half-brother Duke Michael in a desperate attempt to deprive him of his rightful throne. This leads to Rudolf's becoming involved in an elaborate plot to pass him off as the king of Ruritania, thus thwarting the treacherous machinations of Duke Michael and his entourage.
Although the book is entitled The Prisoner of Zenda, it is not the titular prisoner, King Rudolf, who is the most important character; it is his doppelganger cousin. He it is who's perennially at the center of things; of the various plots, sub-plots, and dizzying intrigues with which the narrative is replete. Rudolf Rassendyll is also the quintessential dynamic character, growing into a mature, responsible man over the course of his adventures. This can be seen most clearly in the serious devotion he displays to his duties as king. And even after the real king is finally restored to his rightful throne, Rudolf continues to show how much he's changed by reluctantly parting from his beloved Princess Flavia.