Mary Charlotte, referred to in the series as Queen Mary Charlotte, is the Queen of England and the wife of King George III. She moves within the British court, a position that places her at the heart of the political and social intrigues of the eighteenth-century setting. Her background as a royal consort gives her a degree of influence and access to information, though she is not a central political operator.

Her most notable trait is her prior friendship with Lia de Beaumont, the female identity used by the Chevalier d'Eon. This friendship suggests a personal warmth, loyalty, and a capacity for forming genuine bonds beyond courtly obligation. Her ability to recognize D'Eon when he is in another disguise demonstrates keen perceptiveness and an intimate familiarity with Lia's features and mannerisms that transcends simple social acquaintance.

In the story, she serves a supporting role by providing a crucial moment of recognition that aids D'Eon's mission or personal journey. Her motivations appear rooted in her personal connection to Lia: she acts out of friendship and perhaps a desire to protect or assist someone she cares about. Her relationship with King George III is that of a royal spouse, but the series does not emphasize her political ambitions or conflicts.

Her notable ability is this sharp observation and memory, allowing her to see through a disguise that others might miss. There is no indication of special combat skills or magical abilities; her strength lies in her intellect and emotional insight. She undergoes little personal development within the narrative, functioning more as a reliable ally whose loyalty is fixed from her history with Lia. Her presence provides a rare touch of genuine personal connection amid the larger conspiracies of the plot.