Who is His Serene Highness, Ernst the Prince of Seitz-Bunau ( from Hornblower and the Atropos and what is their importance?

His Serene Highness, referred to as Prince, throughout most of the novel, is the great-nephew of the King of England. He is heir apparent to the crown of the principality of Seitz-Bunau. Unfortunately, Napoleon's conquest of Europe has forced him to flee. Prince is described as intelligent, organized, and mentally acute, but lacking in education.