Mrs Gardner is devoted to the care of her husband who had been invalided since the First World War with shellshock. She described her relationship with her brother Captain Trevelyan as "estranged". Trevelyan had objected to her marriage. He was well off but Mrs Gardner and the other sister had both married poor men. She had seen little of her brother for over ten years. When her husband had been invalided out of the army after the war, she had hoped for a little financial assistance from her brother. This would have enabled her husband to get an expensive course of treatment which would have been of great help. However her brother had consistently refused to grant her a loan. Mrs Gardner acknowledged that her brother was perfectly entitled to refuse but this had caused them to drift apart.
Captain Trevelyan's will left one quarter of his estate to his sister. As a potential heir, and with her financial needs, this made Mrs Gardner a plausible suspect in Trevelyan's murder. However she explained that on that day she had gone out shopping and then to the cinema, coming home about six. This was separately confirmed by her maid Beatrice, and this satisfied Inspector Narracott. However, Emily Trefusis, after examining train timetables, thought it Mrs Gardner might just be able to travel to Exhampton and back in the time, especially since Dr Warren agreed with her that the time of death could be as early as four in the afternoon, although he himself thought it more likely to be about six.
Narracott found Mrs Gardner an intriguing personality. She seemed unnaturally calm and unnaturally ready with her recital of facts. She also did not ask for details of her brother's death. When Narracott offered to tell her more, she asked to be spared "any revolting details" as long as she was assured that the death was painless. She explained her unnatural calm by telling the inspector that, from her husband, she had heard of a great many horrors.
Narracott observed that Mrs Gardner was a woman of iron nerve and had a commanding presence. She had a rather unusual looking face, "broad about the brows, and black hair with a touch of gray at the temples, which she wore combed straight back from her forehead."
Mrs Gardner is the godmother of Ronald Garfield.