Adelaide Becket is an agent with British Intelligence.
More formally and accurately, Becket is addressed as Lady Adelaide Azalea Margaret de Morville. Now that she has married she can also be referred to as Mrs. Hugh Becket even though he is deceased. She prefers the shortened 'Adele'.
The year we first meet Becket is 1906. Edward VII is the reigning King of England, about halfway through his ten-year rule.
We are told immediately in a foreword to the first recorded adventure that Adele Becket was "lately of the Cape Colony" and "was born the daughter of an Earl, but is now the widow of a commoner". We also learn that "she straddles two worlds, speaks fluent German, and can ride, hunt, and shoot".
We are following her actions because "her talents [drew] the eye of spymaster William Melville, who recruits her to help him fight a shadow war against German agents both at home and abroad".
Melville's explanation for why he chose to approach Becket to work for him as an operative made considerable sense, though the last part was not likely to endear him to her when he stated, "You speak fluent German with an upper-class accent, thanks to your three years in Cape Town. You are a peer and a member of society. And you are...well, forgive me for saying so, Lady Adelaide, but you are a woman and are therefore easily discounted in the minds of men - if they notice you at all".
Melville's job was, he said, simple. It fell to him to "root out German spies" to stop them from interfering with British interests and trying to harm or even bring down the British government. When he learns of a plot to go after the man sitting on the throne of England, he knew he needed the help of someone who could watch from 'inside'.
That someone, obviously, he chose to be Becket. She had only just returned from Capetown a few days before. After the death of her husband and small son in a house fire, she had nothing left for her in the Colony where they had lived for the past eight years, so she had returned to England. With little else to occupy her time and considering her former status in high society, she would fit well the requirements of Melville for an inside 'man'.
There is, she finds out the hard way, no real training for her job; operatives were simply told to do something and were expected to do their best and hopefully not get killed. Doing her best is something she is good at. Not getting killed is still up in the air.