Louis Joseph is the second child and first son of Marie Antoinette and King Louis XVI, making him the crown prince of France and the heir to the throne. He is a physically frail boy with blonde hair and handsome features resembling his mother, but he possesses a sharp intelligence and a maturity beyond his years. His personality is charming and earnest, and he deeply admires Oscar François de Jarjayes, the commander of the Royal Guard. He often expresses a sincere desire to become a good king who can improve the country and the lives of its people.

In the story, Louis Joseph plays a supporting role as the beloved dauphin whose presence brings joy and hope to his parents, especially his mother. He shares a close bond with Marie Antoinette and also interacts with Oscar, who takes him horseback riding on one occasion. During that ride, he confides his aspirations to Oscar, revealing his thoughtful and idealistic nature. However, his health deteriorates due to a chronic illness, and he dies at the age of seven, a tragic loss that deeply affects the royal family and marks a turning point in the narrative as tensions escalate on the eve of the French Revolution. His death serves to heighten the personal sorrow of the queen and underscores the fragility of the monarchy. Notable abilities are limited to his precocious intelligence and insightful observations, though his physical weakness prevents him from engaging in strenuous activities.