In the novel Dumb Witness, Mary Tanios is the young daughter of Bella and Jacob, about 7 years old.
Mary is described as a "thin, peaky-looking girl". While Poirot speaks to her mother, Mary writes a letter, which Hastings observes is a "laborious task" for her. She also draws a picture of Mickey Mouse.
Later in the novel, Bella leaves her husband, and takes Mary and her brother Edward (or John) to Wilhelmina Lawson's home. Miss Lawson gets them rooms at the Wellington Hotel, in Queen's Road. Poirot later advises Bella to take the children to the Coniston Hotel near Euston.
While at the Coniston Hotel, a man arrives with a note for Bella. She later comes down with the children and a suitcase, and the children leave with the man.