Harry Nettlefield is an agent with British Intelligence.
The period of time of his activity starts on March 7, 1914, putting the events more than 3 months before the assassination of Archduke Ferdinand of Austria-Hungary while visiting Serbia, and more than 3 months before the first declarations of war would be issued. Tensions all about Europe were extremely hot and intrigue was all about but no open hostilities had yet to start.
When we first meet him, he is gainfully employed as a radio operator on a cargo ship. He describes it as such: "My name is Harry Nettlefield, aged twenty-six, and my profession is that of Marconi operator, possessing a first-class certificate by the Marconi School, and also the Postmaster-General's certificate of the first-class".
He goes on to explain in the first recorded adventure we have of his that his father had been in the British Consular Service so his youth was spent mostly in Germany, Holland, and Italy, giving him excellent command of those languages as well as his native English. He tells us that after he had graduated school he went into the motor-car business but soon found that work somewhat dull so fell back on his love of radio and became a qualified wireless-operator.
It is important to remember that the first practical radio transmitters and receivers were developed around 1895, only twenty years before these adventures, and radio did not start getting used on ships around a decade later so Nettlefield's being qualified makes him one of the early adapters since he tells us that he has been working in this field for two years already.