In the novel Lord Edgware Dies, Captain Ronald Marsh (later 5th Baron Edgware) is Lord Edgware's nephew and heir to his title. He was initially suffering from money troubles, until his uncle's death. He had gone to Eton and Harrow with someone named Spencer Jones.
He is a dark young man, with a round vacuous face, and a small black moustache. Hastings describes his moustache as being "marooned like an island in the middle of the expanse of face".
Captain Marsh had lived in Lord Edgware's house until three years prior to the events of the novel. He had quarrelled with his uncle, and Lord Edgware forbade him the house.
Captain Marsh is an acquaintance of Carlotta Adams, and is with her when Carlotta is invited to Jane Wilkinson's supper party at the Savoy.
When Poirot tells Captain Marsh that Jane Wilkinson was at a party in Chiswick at the time of the murder, Captain Marsh says that at six o'clock she had been "throwing her weight about, declaring that nothing on earth would make her go". He says that such behaviour is "like a woman", and that when planning a murder, one should never depend on a woman doing what she says she will do.
On the morning of the day of the murder, Captain Marsh had called on his uncle, to ask for money. His uncle had refused. In the evening, he dined with the Mr and Mrs Dortheimer, and their daughter, Rachel. He went with them to the opera at Covent Garden, had supper with them afterwards, and danced with Rachel. So, he had an alibi for the time of the murder.
After his uncle's death, Captain Marsh moves into his uncle's house, in Regent Gate. Before that, he had lived at Martin Street, St George's Road.
It is later revealed that Captain Marsh had met his cousin, Geraldine Marsh, at Covent Garden on the night of the murder. They took a taxi to Regent Gate, and asked the driver to wait. Captain Marsh was seen entering the house using a latchkey. He later came out with Geraldine, and they asked the driver to take them back to Covent Garden.
Captain Marsh makes a statement in which he says that he had been "in a hole for money", and needed to get it by the next day. He had met Geraldine at the opera, and told her about it. She had suggested that he take her pearls. Captain Marsh would raise the money he needed on the pearls, and redeem them at a later time.
Captain Marsh also says that when he was waiting by the taxi, he saw a man entering his uncle's house, letting himself in with a latchkey. Captain Marsh was surprised that the man had let himself in with a key, and also thought that he recognised the man as a well-known actor, so he followed the man into the house.
Captain Marsh is later arrested, although Poirot believes that he was telling the truth about his movements on the night of the murder.