Sarah Gillespie is an agent with MI5.
She will be, in time. When we first encounter her, she is a 19-year old single woman living in a small house in Dublin with her widowed father and her sister Maura, younger by two years. It is a life with mixed blessings in that she and Maura were extremely close and loving, even more so since they had to put up with their drunkard of a father, an abusive man that often left welts and bruises on both young women simply because he could especially when he had been drinking which was most nights. She did have the joy of her relationship with young Paul though her father vehemently denouced him; Da would certainly have been against anyone who stood a chance of taking away one of his conventient punching bags.
So life had some bad parts and some good parts and then, on a horrific night when Paul had just told her he was leaving to join the British in the fight against Hitler in May of 1941, an errant raid by the Luftwaffe dropped an enormous amount of bombs on neutral Ireland and one of those ignited a gas main and the resultant destruction took her father, almost a relief in that, but also Maura, a devastating loss. It nearly took Sarah as well and in the weeks of recovery she had to endure, her hatred for Hitler and the Nazis and the whole need to stop them grew to where she felt she had to do something herself.
All this is told in the early pages of the first recorded adventure. As that continues, we watch Gillespie as she leaves Ireland to stay with relative in England and it will be there that circumstances will bring her into contact with representatives of MI5 and the chance to do her part in the war effort.