Le Chevalier D'Eon is one of my favorite things currently. There are many interesting variations between the manga, the anime, and the true history of Charles Geneviève Louis Auguste André Thimothée d'Eon de Beaumont(No, that's not another Vash the Stampede alias).
First(but not by far) there was the manga. The manga takes actual historical figures from 1700's France and turns them on their heads(though D'eon, least of all) by making them quite different from their originals. The manga has more in common with Hellsing than anything else, including the art style and amount of bloodshed. There are Poets who write songs in the blood of innocents and turn into monsters. They usually start out as fairly regular people and sort of get possessed and are apparently lead by a man who resembles Alucard of Hellsing in dress, but would remind D-Gray-Man fans of the MIllennium Earl.
In the manga D'eon gets possessed by the spirit of his sister, Lia, gains a feminine figure and dons a complicated dress, lingerie, button-up boots and wig to become the Chevalier Sphinx who battles the Poets while searching for the reason of her death. She goes into battle with Robin, a young man with muskets, pistols, etc and a cat. The cat, when called upon, merges with the sword wielded by the Sphinx when she calls upon the power of the Egyptian God, Thoth. Which is pretty funny in it's What? Huh? connotations.
Other historical figures(Louis XV, Madame Pompadour, the Comte Saint-Germain and others) are included, usually in a bit more gothy attire than you'd ever find in 1700s France.
The Le Chevalier D'eon anime is much more historically based. In the anime D'eon gets possessed by his sister and his body morphs into hers while she fights Poets and searches for the meaning to her own death. The Poets in the anime version are quasi-alchemists who use Pslams to call upon God for special powers to serve their "righteous cause" and fill dead bodies with mercury so they can control them with alchemical symbols. These zombies are known as gargoyles.
In the anime, King Louis XV sends D'eon and friends abroad to investigate various plots against the monarchy(though D'eon did this alone and as a woman in real life) to Russia and England. The journey also brings D'eon and Lia closer to the answers they are looking for regarding her involvement with the Poets and her death at their hands. Intrigue abounds in the anime version of the story with kingly plots and secret societies(all based on real events and people).
Of course, the true life events are the most fascinating of all and, if you're interested, you can read about the true Chevalier D'eon in this 1885 biography(in PDF format) right here in The Strange Career of the Chevalier D'eon http://books.google.com/books?hl=en&id= ... EnFoX6jTGk