Florence Fairweather is an agent with British Intelligence.
She is already a part of the clandestine service during the still early days of the Second World War - January of 1940, to be exact. War had been declared on Germany by Britain and France three months before but hostilities had not yet erupted; the press often referred to it being a 'phony war'. While some still believe the Chamberlain approach of appeasement was the right way to go, Churchill led what many called the 'war-hawks' who were certain there was only one way to stop Hitler.
A close friend to Churchill and someone already putting in considerable time studying the matter, Fairweather very much shares his beliefs, as does her beloved older husband, Sir Henry, a contemporary of Churchill and the man running British Military Intelligence - although as Churchill points out, it is Florence who starts running things when she comes up with a plan she believes in.
She is an accomplished pilot with many years of training and practice and we watch her fly a Spitfire from France back home to England and Sir Henry in a nasty snowstorm. She is well aware of the problems with those conditions but she is often secure in her abilities. This is a trait she will demonstrate many times in the course of her adventures.
Her reason for being in France during those conditions shows she is already no stranger to playing a personal role in the covert world; she had flown across the Channel in the first place to deliver an important, confidential package on behalf of Churchill. And it is she who will voluntarily "forge a team of female spies and turn them into a fighting force".
Fairweather is a tad over 40 years old, based on the fact she mentions the loss of her first husband during WW1 with her being 18 at the time of the short marriage and his death coming 23 years before the current events. She would remarry to Sir Henry after she allowed herself a decent time to mourn. It will be through her backing him and guiding him that he will rise in influence and authority but now with her country at war, she steps forward.