Who is Antoine Amedee Marie Vincent Amat Manca de Vallombrosa, Marq from The Rise of Theodore Roosevelt and what is their importance?

In the budding town of Medora, Roosevelt meets a tycoon, de Mores. De Mores is a Frenchman who vows to make a name for himself in the Dakota, only to use his money to help finance France to regain land he believes they rightly should possess. The blizzard, though, finalizes the failure of de Mores' beef shipping business and he moves further west, seeking his fortune in mining.