Liz Carlyle is an agent of MI-5.
As the series begins, she has worked for the department for over ten years, having started there just out of school. She is in her mid 30's, single, and getting just a little bored with her routine. She finds small ways to color her life, starting with the clothes she chooses to wear to work. While not technically against the bureau's dress code, the bright colors and odd combinations press the issue and signify her individuality in the otherwise drab corridors of Thames House. This modicrum of rebellious behavior extends to her sometime relationship with a married man, a man whose spontaneity she enjoyed but whom, she admitted, she didn't love. Since her relationship was secret and certain to go nowhere, she routinely had to listen to her mother strive to find a good date for her.
Less than a year before the first book, she had asked for and received a transfer to the Joint Counter-Terror group and as such she attended routinely the meetings between her bureau, MI-6, and the Special Branch, not to mention the Home Office and the Foreign Office when they felt interested. Her normal duties included analysing the international comings and goings of people of interest and information brought her division from a small cadre of informants and undercover operatives. Not the least of her job was studying any and all intelligence to get glimmers of possible dangers to the country.
As the series progresses, she moves yet again in her career, this time to the Counter-Espionage group where she takes on the job of watching for spies. This had been the primary focus of MI-5 for several decades until the rise of terrorism took much of the attention. While that danger still remains the strongest threat, the recent rise in enemy agents has forced a return to their original mandate and Carlyle is one of their best agents.