Who is Armand St. Just in The Scarlet Pimpernel?

Armand St. Just is the younger brother of Marguerite St. Just (Lady Blackeney), a famous actress. Their parents died when they were young. They are very close siblings. While in Paris, Marguerite marries Percy Blackeney, a large man who is considered to be dim-witted, but he is secretly the Scarlet Pimpernel, who saves members of the French aristocracy from death by guillotine. The antagonist is Chauvelin, a French spy, who goes to England to find anyone who is trying to save French aristocrats. Chauvelin uses Armand St. Just to blackmail his sister to try to get to the Scarlet Pimpernel. Armand St. Just is also in collusion with the Scarlet Pimpernel and is a member of the Scarlet Pimpernel League.