In the novel The Mystery of the Blue Train, Armand, the Comte de la Roche is the lover of Ruth Kettering. Eleven or twelve years before the events of the novel, they met in Paris. However, Ruth's father, Rufus Van Aldin, made them break off their relationship.
During the events of the novel, the Comte stays at the Villa Marina at Antibes, which he has leased. He drives a scarlet two-seater car, which has been custom-built, and has a "far more powerful engine than would have been suspected from its appearance".
At the beginning of the novel, the Comte seems to have rekindled his relationship with Ruth, as Van Aldin sees him outside her house at Curzon Street. According to Mirelle, Ruth sees the Comte almost every day. It is not known how true this statement is. However, Mirelle also says that Ruth is going to Paris to meet him, and Ruth later tells Katherine Grey that this is what she is going to do.
According to Commissary Caux, one would not find the Comte's name in the Almanac de Gotha. He is known to the police, but they have not been successful in arresting him. This is because the Comte is cunning, and always conducts his affairs with ladies of high social position, who do not prosecute even if he obtains money from them under false pretences or blackmails them.
The Comte is described as being a "tall dark man, exquisitely dressed, with a somewhat haughty cast of countenance". He has a "fine black moustache". He looks so aristocratic that it seems like heresy to say that his father had been a corn-chandler in Nantes, although that is the truth.
After Ruth's death, a letter from the Comte is found in her handbag. In this letter, he claims to be writing a piece on famous jewels, and to be devoting a section to Heart of Fire. Poirot and the police consider it likely that the Comte was planning to steal the rubies, including Heart of Fire. However, Poirot is of the opinion that the Comte is not the type to commit murder. According to Poirot, the Comte is a swindler who preys upon women, and people of his type are cowards, who take no risks.
When M. Carrège, the examining magistrate, asks the Comte if he met Ruth on the Blue Train on the evening of the 14th, the Comte says that he arrived in Nice on the morning of that day. He later tells his servant, Hipolyte, to tell the police, and anyone else who asks, the same thing.
The Comte later receives a visit from Mirelle, who tells him that the police suspect him of murdering Ruth. She tells him that Derek Kettering is the real murderer, and tells him to go to the police with this information.
After learning that the police suspect him, the Comte drives out and posts a brown paper parcel. It is later revealed that this parcel contained exact duplicates of Heart of Fire and the other rubies. Poirot surmises that the Comte had planned to substitute these for the real rubies.
It is later revealed that the Comte actually arruved at the Villa Marina on the morning of the 15th, not the 14th, as he had said. This breaks his alibi for the time of Ruth's murder.