Comte Robert de Montesquieu was a fin-de-siècle poet and dandy – he was known to wear a white velvet suit with a bunch of violets at the neck in place of a cravat – with plenty of money and an illustrious name (his grandfather was an aide-de-camp to Napoleon). He had lots of important friends – Alphonse Daudet, Sarah Bernhardt, Eleanora Duse, Gabriele D’Annunzio, Luisa Casati, J.A.M. Whistler, Jean Cocteau. But he lives on most as the model for two of the most famous dissolute characters ever created in fiction: Proust’s Baron de Charlus and Huysmans’ Des Esseintes.
Montesquieu died in 1921 at age 66.
He has a page on wikipedia here. Alain Delon played the part of Charlus in Volker Schlöndorff’s Swann in Love, styled to look very much like Montesquieu and appearing briefly in a peculiar German trailer for the movie here.