Prince Franz Charming is the prince of a neighboring kingdom, sometimes called Goldland. Orphaned young, he was raised by his uncle, who serves as his guardian and pressures him to meet royal duties, including political marriage alliances, creating Franz's conflict between obligation and personal desire.

His core narrative involves romance with Sapphire, the protagonist raised as a prince. Franz interacts with Sapphire through three identities: as Prince Sapphire (a male rival he initially distrusts due to misunderstandings), as the masked Phantom Knight (seen as a romantic competitor for a princess), and as Sapphire's disguised female persona (a blonde princess he deeply loves). Unaware these are the same person, his love for her female guise drives key plot actions, including protecting her from antagonists.

Franz displays athleticism and swordsmanship, though Sapphire is often shown as slightly more skilled. He exhibits loyalty and courage, risking his status to protect Sapphire, such as agreeing under duress to marry the demon Hecate to save Sapphire's life, resulting in disownment by his uncle. He confronts antagonists like Duke Duralumin, Madame Hell, and Mephisto/Satan, acting as Sapphire's persistent ally.

In the sequel *The Twin Knights*, Franz marries Sapphire and becomes king. They have fraternal twins, Prince Daisy and Princess Violetta. When changed inheritance laws spark debate over succession and Duchess Dahlia kidnaps Prince Daisy, imprisoning Franz, Sapphire, and Violetta, Franz supports Violetta's escape and training to rescue her brother, affirming his role as protective father and monarch.

Franz appears in the original 1953-56 *Shojo Club* manga, the 1958-59 sequel *The Twin Knights*, and the 1963-66 *Nakayoshi* manga remake, consistently portrayed as Sapphire's primary love interest navigating political and supernatural conflicts.