Armand St Just's girlfriend

Sort of. In Eldorado, he meets an actress called Jeanne Lange, but she isn't Marguerite's friend or understudy. He does betray the Pimpernel to save her, though (or so he thinks).

"Tony, if you talk that rubbish, I shall be forced to punch your head" - Lord Tony's Wife, Orczy

The only Louise I have come across in the original book is Marguerite's maid.