Freya Matthews is an agent with British Intelligence.
In this two-book series written in the mid-to-late 60's, she is about as independent and liberated as any female might want, especially for that era. She has no qualms about travelling to various parts of Europe, be it Italy or Spain or Greece, on her own or in the company of others.
She was not always an agent. In the first book, she is a marine explorer interested in an ancient shipwreck off the coast of Greece. She had been offered the use of a small yacht belonging to the brother of her former fiancée and she was in the middle of her exploration when life, and her career, took a turn.
The brother in question was Guy Plant, an extremely personable man of early middle age who offered a platonic relationship that she welcomed. As far as she knew, he had little else to do and she needed a ship and it seemed fortuitous.
On Plant's part, the offer of the ship's use was a perfect cover for his real activity, namely a major agent with the British Secret Service working on a case in that region. It was not supposed to turn violent nor had he ever considered using Matthews as anything other than the façade. Things worked out differently than either expected.
In the first book, Matthews could best be described, at least in the beginning, as 'fiercely' independent, to the point that she was often bordering on rude and petulant when anything remotely suggested someone might help her, or worse, that she might need help. And her opinions are took starkly stated for some peoples' liking. Prickly is a good word to describe her.
She mellows out considerably as the story develops and in the second book there is no trace of the hardness, just the self-reliance.
Guy Plant is an important part of the first book but less so in the second as Matthews moves from an ad hoc assistant to a fully functional agent.