As his title implies, the ruler of the European kingdom of Caronia.
Charles is the son of King Charles VIII of Caronia and Queen Helena of Ruritania. His half-brother is Prince Basil the prime minister and his betrothed is Princess Marta of Esterhagen.
He is seen only briefly in his first appearance, just before being shot by KAOS agents working with Basil, who is trying to usurp the throne. However, it is long enough to perceive his remarkable resemblance to Maxwell Smart. As a result, Smart takes his place while he recovers for his coronation [Episode #74: "The King Lives?"].
The following year, Charles visits Washington to secure a loan from the United States to keep Caronia's army strong. For security he brings the Royal Diamond Scepter which the now-deceased Basil's aide, rotten, ruthless Rupert of Rathskeller, is determined to steal to prevent the loan from going through [Episode #106: "To Sire, With Love, Part 1"].
While staying at the Smarts' apartment, Charles inadvertently causes friction in their marriage; 99 cannot tell the two of them apart and Max becomes jealous when he sees her mistakenly kissing the King.
This is no sooner sorted out when Charles is kidnapped and held hostage for the scepter in a secret hideout below the Caronian embassy. Max follows and engages Rupert in a duel, saving the King's life. In gratitude, Charles tells Max that if there's anything he can do for him all he has to do is ask. Max requests that on his next visit "would you mind staying in a hotel? [Episode #107: "To Sire, With Love, Part 2"].
Portrayed by Don Adams (uncredited).