Who was Marguerite St. Just?

Lady Blakeney, while living in France as an actress, was famous for her beauty, but even more for her charisma, wit, and intelligence. When she marries Percy Blakeney, no one's quite sure what she's thinking. Since he's considered a dull turkey and she a renowned socialite, the consensus is that she's married down. But she discovers her husband's secret identity and we see her husband's heroism through her eyes.