The Sieur d'Eon was an 18th century French diplomat and spy whose gender was a matter of contention among political circles. D'Eon was skilled with swords and familiar with Faction Paradox. (AUDIO: Sabbath Dei)
D'Eon studied under the Count Saint-Germain but quit when he began claiming to be over a thousand years old.
D'Eon once "masqueraded as a woman for months on end... fool[ing] the whole of the Russian court".
In 1762, the Sieur d'Eon attended the Hellfire Club's ball at Medmenham Abbey. There, d'Eon investigated the site of Mary Culver's ritual and was followed by Sabbath. They met Cousin Justine and witnessed her timeship rise from the ground. Later, d'Eon met Mary Culver and was enlisted, along with Cousins Justine and Eliza, to stop Lolita's plot. (AUDIO: Sabbath Dei)
Justine was revulsed by the idea of a man masquerading as a woman or vice versa (despite herself carrying the Grandfather's shadow); likewise, d'Eon mistrusted Justine for being a self-appointed Messiah. However, d'Eon worked well with Eliza, and together with Lord Sandwich, they destroyed Lolita's army of 500 Peking Homunculi under Buckingham Palace. (AUDIO: In the Year of the Cat)
D'Eon later visited Slough where they had a fencing match with the Tenth Doctor. The Doctor and Rose Tyler travelled to London with them. They went with Rose to find Joxer and Hempel before she was captured to be a slave. They then fought Joxer's copies of the attendants so that the Doctor could get to the ship. On board, they suggested a fencing match against the slavers. (AUDIO: The Sword of the Chevalier)