Elsa Hardt

In the short story The Adventure of the Cheap Flat, Elsa Hardt was an American concert singer and also believed to be an accomplished international spy who had "done much nefarious work under various aliases." The American authorities believed that she had taken a set of naval plans which been stolen by Luigi Valdarno and who was later found dead in the East Side of New York.

Elsa and her accomplice (husband or brother, the relation was never made clear) was believed to have fled to London hoping to sell the plans to Japanese contacts. While in London, Elsa and her accomplish took a flat under a cover name of Mr. and Mrs. Robinson (likely chosen because it's a very common name). Elsa later learnt that an assassin of an Italian secret society was looking for them (to avenge the death of Valdarno who was a member) and she and her accomplish contrived to sublet their flat to another couple named Robinson as a decoy. They offered the flat at a low rent and after turning down many visitors, they managed to rent it out to John and Stella Robinson. The assassin would kill the innoncent couple, leaving the society satified in the belief their revenge has been fullfilled, and Elsa Hardt and her accomplish would have escape their doom. But Poirot suspected foul play upon hearing about the low rent on the flat, and managed to save John and Stella Robinson and bring Elsa Hardt to justice.

Portrayals
Elsa Hardt was played by Jenifer Landor in the film adaptation of The Adventure of the Cheap Flat in Series 2 of ITV's Agatha Christie's Poirot. Here her real name is given as "Carla Romero" and her backstory is changed and embellished significantly.

The secret papers were not details of harbour defences but the plans of a new design of submarine. Her prospective buyers were not Japanese but the Italian government. Luigi Valdarno was not a member of an Italian "Brotherhood". It was Carla herself who was working for the "Brotherhood".

Carla's role was to act as a temptress whose job was to persuade Valdarno to steal the plans and then take them and hand them to the "Brotherhood". However she decided to doublecross them and flee to London under the cover name of the Robinsons. After installing another Robinson couple as a decoy, Carla then took on another cover name of Elsa Hart (not Hardt), a Canadian singer who worked at the "Black Cat Cabarat".

Miss Lemon was sent in undercover to interview Elsa and verified that she was not Canadian but American and had come from New York around the time Luigi Valdarno had been killed. Her claim to have been on tour in Europe for a long time was not credible. She said she had gone stag hunting in the Bois de Boulogne which was impossible. Miss Lemon also spotted a musical score in her dressing room which could only have been bought in New York six months ago.

In the NHK's anime adaptation The Riddle of the Cheap Flat, the character of Elsa Hardt (called Elsa Hart here) is voiced by Rica Fukami. The portrayal here is very close to the original story. She also has a companion, here identified as her brother, voiced by Tetsu Inada.