Of the characters, Flora is cute, and Caroline has a nice presence. It feels like the prototypes of characters that would appear in Christie's later works can be seen here.
However, her character hasn't been fully established yet, and she's a little reserved. Caroline's character would probably be more aggressive in later works. But Caroline in this work has a bit of refinement. I feel like that would be good in its own way. And above all, Poirot is cool...no, scary! He shows a different side of us from previous works. The interactions between Poirot and Dr. Shepherd are calmer than those between Poirot and Hastings...or rather, they feel awkward. And this gives the story a more serious atmosphere. In fact, Poirot in this story is stern and scary. The final resolution is especially terrifying. It's horror. The scene where the culprit is identified makes your heart skip a beat, and then you break out into a cold sweat. You might even notice this more when re-reading the story. I think the way Poirot coldly exposes the lies of the characters and the criminal's fraudulent schemes is excellent.