Sapphire hails from the Kingdom of Silverland, born with both a male blue heart and a female pink heart due to a celestial error by the angel Tink during her birth. Tink accidentally provided an extra heart, contravening divine intention. Silverland's inheritance laws prohibiting female rulers forced her parents to conceal her biological sex and publicly raise her as the male prince, preventing the antagonistic Duke Duralumin from seizing the throne. This required rigorous training in masculine skills such as swordsmanship and horsemanship, alongside covert education in traditionally feminine arts.

She navigates a fluid identity through multiple personas: the public "Prince Sapphire" heir, the masked vigilante Phantom Knight combating injustice, and privately, a princess wearing gowns and blonde wigs. This multiplicity stems from her biological reality and societal constraints, leading her to actively perform both masculine and feminine roles contextually. She demonstrates athleticism, courage, and combat proficiency while embracing activities coded as feminine within her medieval setting. Sapphire consciously amplifies masculine traits like aggression or competitiveness when her legitimacy as prince faces scrutiny, engaging in duels to deflect suspicion. Conversely, her princess persona emphasizes beauty and grace, notably captivating Prince Franz Charm during a disguised encounter. Despite pressures, she asserts autonomy over her identity, refusing to relinquish either heart to Tink until she personally determines her gender.

Sapphire's narrative extends beyond the core *Princess Knight* manga and its 1967–1968 anime adaptation. In the 1980 *Astro Boy* episode "The Time Machine," she appears as the 15th-century Prince of Molavia. A time-travel incident gravely injures her, leading surgeon Black Jack to discover her forced male presentation during life-saving surgery. This culminates in her publicly affirming her female identity and ascending the throne as princess, aided by Black Jack’s intervention repealing the kingdom’s male-only succession law. Later, in the 2003 *Astro Boy* series episode "Astro Vs. Atlas," she has a brief non-speaking cameo as a scientist employed by the Ministry of Science, indicating a career shift in a futuristic setting. A theatrical short animation for Kyoto’s Tezuka Osamu World reframes her story as a metaphor for perseverance against adversity, with the Phoenix comparing her resilience to historical figures like Minamoto Yoshitsune.

Visually, Sapphire has short brown or black curly hair, often styled with early 20th-century American fashion influences, and large, shimmering eyes. Her primary prince attire includes tights, a brimmed hat, and a jacket adorned with a distinctive ribbon pattern. As princess, she dons ornate gowns and golden blonde wigs. As the Phantom Knight, she obscures her face with a mask and wears practical fencing gear, highlighting the constructed nature of her gendered presentations.

Key relationships define her journey: Tink evolves from a celestial agent tasked with retrieving her extra heart to a loyal, albeit often ineffectual, companion. Prince Franz Charm becomes a romantic interest, his perception shifting based on her presentation—viewing her as a rival prince, a love object princess, and an adversary Phantom Knight. Antagonists like Duke Duralumin and the supernatural Mephisto (Satan) persistently threaten her position, with Mephisto coveting her female heart for his daughter Hecate. Despite initial alignment with her father, Hecate ultimately aids Sapphire covertly.