Louis Philippe II, the duc d'Orleans comes immediately to mind. During the reign of his cousin, Louis XVI, he was very outspoken against the monarchy and favored reforms within French government that could be considered pretty leftist for the time. His aim was not necessarily to overthrow the monarchy, but to institute a constitutional monarchy along the same lines as England. Orleans' support of the nascent revolutionary movement in the late 1770s and 1780s was complicated but significant. He funneled huge amounts of money into the anti-Royalist movement, but like I said, his aim seems to have been to overhaul the existing system in French government rather than just abolish the monarchy entirely. He publicly funded and helped protect libellists who attacked the King & Queen. Of particular interest to my area of study, Orleans was directly responsible for a significant portion of the pop culture narrative of Marie-Antoinette that still persist to this day, as a result of these libelles.

In any case, when the Revolution took off in the early 1790s he was sure to publicly ally himself with Robespierre in an effort that probably had more to do with trying to save his own neck than any real agreement with the Jacobins. He renounced his hereditary title and adopted the new name Philippe Égalité. Didn't help, though because he went to the guillotine almost exactly 1 month after his rival Marie-Antoinette did.