Miss Pierce is described as a "vague little middle-aged lady with wisps of grey hair". She is often described as "twittering" when she speaks.
Miss Pierce had worked for years as a nursery governess. She tells Poirot that at the time of the case of the ABC murders, she had a post as a goveress near Doncaster. Some time before the events of the novel, she had suddenly come into a small legacy, which enables her to fulfill her lifelong desire to travel.
Miss Pierce is afraid of heights, and while at Petra, she has to be "almost carried over the precipitous places". According to her, she has suffered from this fear since childhood, and she "never could look down places". Dr Gerard helps her by holding a stick between her and the sheer drop. She finds that this illusion of a rail helps to overcome the feeling of vertigo.
While at the camp at Petra, Miss Pierce reads a book entitled The Love Quest, which is "described on its wrapper as a thrilling tale of passion and misunderstanding". She also embroiders a table-mat. She occupies a tent on the right side of the marquee.
Poirot interviews Miss Pierce together with Lady Westholme. They tell him that on the day of the murder, they rested in the afternoon, and then went for a walk. They called up to Mrs Boynton to ask if they could get her anything, and she only grunted in response. They also tell Poirot that Mrs Boynton had been angry with one of the Arab servants, and had called out and shaken her stick at him.
After Lady Westholme leaves the room, Poirot asks Miss Pierce if she remembered him sneezing when she first entered the room, and she says that she does. However, Poirot did not sneeze. This leads him to believe that Miss Pierce is an unreliable witness, as she is very suggestible.
Miss Pierce later suggests to Poirot that Ginevra Boynton might really be royal, perhaps the daughter of the Grand Duchess Tatiana. She also suggests that Mahmoud might be a Bolshevik agent, or that Sarah King might be a Communist. She then says that she saw something peculiar, which she feels she ought to tell Poirot about.
Miss Pierce tells Poirot that the morning after Mrs Boynton's death, she saw someone, whom she thought was Carol Boynton, throw something into the stream, and that object glittered in the sunlight. She later discovered that the object was a metal box containing a hypodermic syringe. Sarah King said that it was hers, and Miss Pierce gave it to her.
Poirot tells Miss Pierce that what she has just told him may not be important in itself, but it completes his case. This causes Miss Pierce to be "as flushed and pleased as a child".