Abigail Marshall is an agent with British Intelligence.
The time period we follow her is at the end of the Second World War and just after. The organizations she will be involved with are MI9 during the conflict and MI6 when the War ends and her previous employment dissolved.
Born Abigail Linneman, she was studying linguistics at Cambridge before taking time off to work as a waitress in the kitchen of a pub just outside of RAF Witchford, serving the hundreds of young men involved in the War. Within a short time, though, her abilities with languages would become known to the Royal Corps of Signals and she would be quickly made part of the WAAF. Her skill with German came naturally to her from her now deceased German mother and several long visits to Germany before the war to stay with her Aunt.
Most of Marshall's early work for MI9 will be in the form of translating intercepted communications. Her field work will come when her newly wed husband, RAF rear gunner Sergeant James Marshall, is shot down over Germany during a bombing raid and he is taken prisoner, sent to the same Luft-Stalag from where a large number of British prisoners had just escaped - most of them recaptured or killed but still causing much turmoil for Germany (as recounted in The Great Escape).
It is when Marshall decides to make use of her language skills and familiarity with the region from her visits before the War and do her best to rescue her husband that she turns from a translator to a field operative. It will by that success, combined with that language skill and geographical knowledge, that will get her invited to work with MI6 after MI9 is disbanded.
Marshall is described as quite attractive (though not beautiful in her own eyes) with long auburn hair. She is in her mid-20s when we first meet her.