For someone who hasn't yet made a physical appearance in any of the books to date, Cressida Fox (born Cressida Fuchs, brother of Rev Colin Fuchs) casts a remarkably long shadow over the events portrayed in them.
Here's a list of the reasons why.
She was married to the criminal financier Rufus Fairbanks. In fact, it was her money that allowed him to expand his business to the extent that he attracted the attention of the Belarusian mafia. Following an unfortunate encounter with a food mixer, she is now his widow.
While Archimedes and Pythagoras Vavasor were working for Fairbanks, she had an affair with Archie. She also had a brief fling with Pye, although it is still not known whether or not she was aware of this. Either way, it was the discovery of a billet doux in the form of a greetings card signed by Pye and addressed to 'The Vixen' that led to Archie committing fratricide.
She was in the habit of buying copies of George Burgess's work Shroud of Ecstasy in bulk in order to wind up her brother Colin (the unwilling dedicatee of the book) by distributing them to her friends. Inevitably, she gave one of them to Archie, who passed it on to his younger brother Isaac, which is how he came to have the idea of employing Burgess to write a misleading biography of the twins.