Susanne is Daisy Armstrong's nanny.

Susanne was the French nursery-maid employed in the household of Colonel Armstrong and his wife Sonia Armstrong in New York, where she was devoted to their three-year-old daughter, Daisy Armstrong. She was a pretty, laughing girl who looked after Daisy's clothes and acted as a personal maid to the child. The entire household, including Susanne, adored Daisy, who was described as a sweet, happy child with lovely curls. Susanne's role in the Armstrong home placed her in close daily contact with the family, and she was deeply attached to the little girl.

The Kidnapping and Its Aftermath

When Daisy Armstrong was kidnapped and later found dead despite the payment of a two-hundred-thousand-dollar ransom, the police investigation focused intensely on the household staff. Susanne, as the nursery-maid, came under particular suspicion. The police were convinced that she had some knowledge of the crime, believing she had chatted idly with someone and given information about the timing of Daisy's outings. They refused to believe her hysterical denials of any involvement. The pressure of the interrogation and the public suspicion proved unbearable. In a fit of despair, Susanne threw herself from a window and was killed. It was proved afterwards that she had been absolutely innocent of any complicity in the crime. Her death added a fourth victim to the Armstrong tragedy, following the murder of Daisy, the stillbirth of Mrs. Armstrong's second child and her own death from shock, and Colonel Armstrong's suicide.

The Connection to the Conspiracy

Susanne's father was Pierre Michel, a Frenchman who worked as a Wagon Lit conductor for the Compagnie Internationale des Wagons Lits. Her death gave him a powerful personal motive for revenge against Cassetti, the man who had orchestrated the kidnapping and escaped justice. When the conspirators—a self-appointed jury of twelve people connected to the Armstrong family—planned to execute Cassetti (now travelling under the alias Samuel Edward Ratchett), they recruited Michel. His position on the Simplon Orient Express was crucial to the plot: he could ensure that the murder took place on his train, in his coach, and that the timing and logistics were controlled. Michel's participation meant that the conspiracy numbered thirteen people, not twelve, with the thirteenth being the one person who was not a direct member of the Armstrong household but whose daughter's death had been caused by the same criminal. The choice of the Orient Express as the stage for the murder was thus directly linked to Susanne's fate.

Narrative Significance

Susanne's story functions as a powerful emotional and moral catalyst within the novel. Her suicide, an indirect consequence of Cassetti's crime, demonstrates the far-reaching devastation caused by the kidnapping. It transforms the crime from a personal tragedy for the Armstrong family into a collective wound that draws in servants and working-class people, giving them an equal stake in justice. The revelation that Pierre Michel is Susanne's father provides the final piece of the puzzle for Hercule Poirot, allowing him to understand the full scope of the conspiracy. Susanne's name is invoked by Countess Andrenyi (Helena Goldenberg, Mrs. Armstrong's younger sister) when she recalls the tragedy, and by Linda Arden (Mrs. Hubbard, the matriarch of the family) in her confession, where she describes Susanne as a pretty, laughing girl who was devoted to Daisy. Her death is not merely a footnote but a foundational injustice that the conspirators, including her father, seek to avenge.