Mr. Harley Quin

Harley Quin is a fictional character created by Agatha Christie and the most mysterious of all her detectives. His name is a word play on "Harlequin", which is a clue to his personality and his very existence. Quin helps his old acquaintance, whom he visits from time to time, Mr. Satterthwaite, a social dilettante, solve crimes using his extraordinary skills and instincts. He appears in the 12 short stories appearing in "The Mysterious Mr Quin", first published in 1930, and in an additional two short stories, "The Love Detectives" and "The Harlequin Tea Set" from "Problem at Pollensa Bay and Other Stories". Satterthwaite, who appears together with Quin in all the previously mentioned short stories, also appears without him in Christie's short story "Dead Man's Mirror" as well as in her novel "Three-Act Tragedy".
In her , Christie wrote that Quin and Satterthwaite (whom she called Quin's "emissary") became her favourite characters. The reason was that she did not exploit them as much - she refused to make series of books about them, writing only when she felt like it. Here is how she described the protagonist: "Mr Quin was a figure who just entered into a story – a catalyst, no more – his mere presence affected human beings. There would be some little fact, some apparently irrelevant phrase, to point him out for what he was : a man shown in a harlequin-coloured light that fell on him through a glass window; a sudden appearance or disappearance."