Lady Smallwood is the widow of Lord Smallwood. She was part of an inquiry into the interactions Charles Augustus Magnussen had with the British Prime Minister. During the course of the inquiry, he used correspondences between Lady Smallwood's husband and his former lover, a fifteen-year-old girl, as blackmail. Her husband later committed suicide.
During this encounter Magnussen proclaimed that he owned her, intending to use the blackmail to control her, and showed her that she was powerless by licking her cheek. Feeling shocked, disgusted and violated, she deviated from her normal route home and traveled to Baker street to beg for the help of Sherlock Holmes.
Sherlock took on the case, as he hated people like Magnussen who pry on people who are different, and attempted to make a deal on Smallwood's behalf for the letters. This was unsuccessful and so Sherlock tried a number of ways to get the letters including breaking into Magnussen's London offices.
Lady Smallwood was instrumental in approving Sherlock's exile, saving him from imprisonment, discussing with Mycroft Holmes a suitable course of action, and was made aware when Jim Moriarty's face was broadcast over the whole of the United Kingdom.
Lady Smallwood appeared again in "The Six Thatchers" as part of a secret government committee that issued a D notice classifying Magnussen's death as an accidental death due to a police marksman with an "itchy trigger finger". Whilst not happy with the murder being covered up, she agreed so that Sherlock was not jailed for the murder, and would be able to focus on Moriarty's presumed return.
She appears to be attracted on some level to her coworker, Mycroft. In "The Lying Detective, Lady Smallwood gives Mycroft her personal phone number and asks him to call her. He acts indifferently to her flirtation, but she does not seem upset by this.
Lady Smallwood is a confident, powerful woman, and she is not easily shaken. She is extremely professional at her job.
- She wears the perfume Claire de la Lune.
- She, Mycroft and a few colleagues are considered to be the most high-ranking people in the Government; similar with Mycroft, her priority level is 'Ultra', and was assigned a codename, 'Love'.