In the world of Le Chevalier D'Eon, the Comte de Saint-Germain stands as one of the most enigmatic and powerful figures, operating from a shadowy position that places him in contact with nearly every major faction. He is presented as a high-ranking Poet, often described as the head of the European poets, and an associate of the Duc d'Orleans, though his true allegiances remain deliberately obscured. His involvement in the central conflict predates the main events of the story, as he was present in England during a crucial soul ritual alongside figures such as the English poet Dashwood, which is suggested to be the origin of the sister Lia de Beaumont's ability to inhabit her brother's body.

Saint-Germain's personality is defined by his profound mystery and inscrutable motives. He appears to be a master manipulator who plays all sides against each other, showing no permanent loyalty to any single nation or master. He interacts with a wide range of characters, from the French court to the lowest poets, often appearing to help one side while secretly aiding another. This is most clearly seen when he assists the young poet Emma, elevating her to the seventh rank and gifting her a daemon of light, while simultaneously ordering her to follow the commands of his apparent enemy, Roble. He also engages with powerful historical figures like Madame de Pompadour, revealing he predicted her rise to power twenty years prior. These actions suggest a long-term, almost inscrutable agenda that has spanned decades. A significant undercurrent to his character is the suggestion that he is immortal and possesses immense power, which may have become a burden. Some interpretations suggest that his seemingly contradictory actions are driven by a deep-seated hatred for his own immortality and a desperate hope that a powerful Poet like Lia or Roble might finally be able to end his eternal life.

Within the story's role, Saint-Germain functions as a crucial catalyst and a puppet master. He is the one who bestows high poet ranks and powerful daemons, directly shaping the abilities and threats that the protagonists must face. He is not a passive observer but an active force who directly confronts major characters, including an attempt on Queen Marie, though his plans are often thwarted due to underestimating his opponents. His key relationships are defined by manipulation and mysterious alliances. He is allied with the Duc d'Orleans, works with figures like the Count of Cagliostro and Lorenza as his subordinates, and maintains a complex, parallel alliance with the English poet Dashwood. In the manga adaptation, his role is more directly tied to the Marquise de Pompadour, who hires him to investigate an illness, creating immediate suspicion and antagonism from other characters.

Saint-Germain's development throughout the story is less about personal change and more about the gradual revelation of his true nature and capabilities. He consistently demonstrates an ability to survive what would be fatal injuries. He survives a confrontation with Queen Marie and Maximilien, endures being crushed by rocks, and even possesses the supernatural ability to transform into a Gargoyle, a monstrous creature typically controlled by other poets. His defeat ultimately requires the combined power of King Louis XV and the combined spirits of D'Eon and Lia, underscoring his status as a final, formidable obstacle. His notable abilities are vast and varied. As a master poet, he can bestow poet ranks and bind souls to create daemons of light, as he did with Emma and her sister. He possesses prophetic powers, having accurately foretold the future for Lady Pompadour. He is an alchemist and a master of artifacts, such as the magical ring and the cup of Melchior. Furthermore, he has the power to issue orders through a magical journal, where his written words appear simultaneously in the copies held by his followers. His ultimate power is his apparent immortality and the ability to transform his own body into a Gargoyle, making him a nearly unstoppable supernatural entity.