Sarah Goldstein is an agent with British Intelligence.
Unofficially.
She is also unofficially working for them when she becomes Ursula Haller.
That status is because when she first starts working with the Captain (more on him in a moment) Goldstein is a teenage girl of Jewish lineage just trying to survive in a Germany which has been her home her whole life. Well, some of that time was lived in Austria when she and her actress/entertainer mother fled after life in Nazi Germany had become untenable. Then the Anschluss brought that country under German control and things got even worse. Her mother's once highly successful stage career quickly died and day-to-day survival became a constant struggle and her mother sank under the pressure and young Goldstein had to grow up a lot faster than most teenage girls.
It was while they were trying to flee into Switzerland using their old car which had been confiscated from them and which her mother had stolen back (they took the car but not all the keys) that tragedy struck hard and in the first page of the first adventure, we find Goldstein the only survivor of a car crash that came as a result of their fleeing a roadblock and her mother being shot and dying.
Trying to get away from the pursuing soldiers, Goldstein is helped by a stranger who she learns was also on the run and in a sequence that is fascinating to read, the two end up helping save each other and Goldstein becomes temporarily connected to Helmut Haller, successful seller of radio sets throughout Germany and Austria.
That is when Goldstein learns that Haller is Captain Jeremy Floyd of British Intelligence who has been living under the Haller cover identity for years and who is on the track of a very dangerous weapon and like it or not (and he does not) he could use someone of her abilities to accomplish his mission. And Goldstein is one frightened but immensely angry young woman who wants to something, anything, to destroy the people who destroyed her life.
And those abilities are impressive. As she explains it, when the laws in Germany changed and her mother was not allowed onstage and couldn't work, "We'd lost all our money in the crash, so she schooled me herself. Languages, accents, acting, listening to records of speeches". Learning languages because her mother was quite gifted and teaching helped keep her from thinking. "Polish, Czech, English, French, Dutch, even Russian." And Goldstein read extensively since she was not allowed in school. She was not trained as a spy but she had a lot of the requisite skills from having to sneak out at night and find food and other necessities.
Goldstein was what the Captain needed and he provided what she needed. Neither could forget, however, that she was still only 15 years old.