Hercule Poirot and Jane Marple are the most prolific detectives in Agatha Christie's (pictured here) stories, but there are others, including 3 of these 4. Which is not one of her characters?

Hercule Poirot and Jane Marple are the most prolific detectives in Agatha Christie's (pictured here) stories, but there are others, including 3 of these 4. Which is not one of her characters?
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