Stella Steele is an agent with British Intelligence.
She is not an official one and she most definitely would not have been acknowledged as such should it come up but she works on their behalf nevertheless and what she works on is her father.
When we meet her, she is around twenty-two years old and is described as having "a sweet, eminently English-looking face" with a "fair" complexion. To much of England and certain most of the theatre-going public in London, she is known as Stella Steele, the famous entertainer "whose picture postcards were everywhere". To her doting father, Theodore Drost, though, she was simply Ella.
Though he saw only his little girl, he had indulged her "childish aptitude" when she was younger and allowed her to pursue her passion. She soon became "a dancer on the lowest level of the variety stage, a touring company which visited fifth-rate towns. Yet, owing to her discovered talent, she had at last graduated through the hard school of the Lancashire "halls", to what is known as the "syndicate halls" of London. It is further stated in the first adventure that "From a demure child-dancer at an obscure music-hall in the outer suburbs, she had become a noted revue artiste, a splendid dancer, who commanded the services of her own press-agent, who in turn commanded half-a-dozen lines in most of the London morning papers, both her prestige and increased salary following in consequence."
When she is not entertaining the crowds in the theaters, though, she is often seen around her father and his friends. Daddy, Theodore Drost is described and thought of as a "grey-haired Dutch pastor." A man "with a curiously triangular face, a big square forehead, with tight-drawn skin and scanty hair, and broad heavy features which tapered down to a narrow chin that ended in a pointed, grey, and rather scraggy beard". Though he was believed by most to be Dutch, he was in fact very much German, Prussian to be specific, and living undercover in London while spying on England for Germany. More dangerously, he was, at the time of the adventures, about to take a much bigger role in the war, moving from agent to agent-provacateur. In modern parlance, a terrorist. Helping greatly in this nefarious endeavour is the fact that for many years Drost had been a professor of Chemistry and that makes bomb making an easy task.
To Drost's friends, Ella/Stella was a danger. She listened too carefully and watched too closely what her father and they were up to and she was known to spend time around an Englishman they suspected of spying on them. Daddy was blind to the possibility of her working against him for she was "too good a daughter of the Fatherland". He was very much wrong for he thought her allegiance, like his, was to Germany but her mother had been English and she had been raised largely in the United Kingdom especially after Drost was sent as a sleeper agent by the Kaiser more than a decade before. She was in heart and mind British and never once considered going along with her father.
Chief among those highly suspecting of Ella, though, is Count Ernst von Ortmann, "a trusted official in the entourage of the Kaiser, and having lived his early life in England, being educated at Oxford, he was now entrusted with the delicate task of directing the advance guard of the German army in [England]." Several times, just before each mission which would be thawrted by Stella and her boyfriend, Seymour Kennedy, an officer in the Royal Navy and part of the intelligence community, von Ortmann would issue stern warnings about them both to Drost but each time they fell on loving but naive ears.