Harley Quin is a character who appears in various short stories published through the 1920s. These were subsequently collected and published for the first time in The Mysterious Mr. Quin in 1930 and in two other short stories: The Love Detectives and The Harlequin Tea Set.
In all the stories Harley Quin helps his old friend Mr Satterthwaite to solve crimes through carefully worded but apparently random observations and questions that provide insight into a problem. Mr Harley Quin is there to act as a catalyst, to reveal to Mr Satterthwaite what he already knows. As Satterthwaite would come to expect, Harley Quin often appears "in the nick of time" and whenever he appears, Satterthwaite knows there is some impending problem although what the problem is is not always apparent at first. Mr. Satterthwaite is always happy to encounter his elusive friend because it is on those few occasions that he ceased to be an observer and becomes an actor in the theatre of life.
Who or what Harley Quin is continues to be a mystery. Even Satterthwaite would admit that despite having known him for a long time he doesn't really "know" him. Harley Quin appears and disappears unexpectedly and by strange tricks of light and reflections; making his clothes seem brightly multicoloured. This aspect of Harley Quin is based upon Harlequin from the 16th century Italian Commedia dell'arte. The earlier versions of the character were that of a clown or fool although in the 18th century the character changed to become a romantic hero. It is in this guise that he more closely approximates Christie's character of Quin, who in most of the stories helps unravel emotional entanglements in addition to solving the crimes and mysteries.
In The Coming of Mr Quin, Quin tells Satterthwaite "I must recommend the Harlequinade to your attention. It is dying out nowadays – but it repays attention, I assure you." The Harlequinade was the still-later British stage version, in which Harlequin has magical powers, and brings about changes of scenery by a touch of his slapstick.
In the various stories, several impressions are given to the reader through Satterthwaite's almost subconscious thoughts of the connection between the appearance of Quin and the traditional costume of Harlequin, the latter being a dark mask and clothing composed of multi-coloured diamond-shapes as featured on the cover of the UK first edition of the book (see image above):
- The Coming of Mr Quin - The first time Harley Quin is seen is described in the following passage: :"Framed in the doorway stood a man's figure, tall and slender. To Mr Satterthwaite, watching, he appeared by some curious effect of the stained glass above the door, to be dressed in every colour of the rainbow. Then, as he stepped forward, he showed himself to be a thin dark man dressed in motoring clothes."
- The Coming of Mr Quin - On the mask: "Mr Quin acknowledged the introductions, and dropped into the chair that Evesham had hospitably pulled forward. As he sat, some effect of the firelight threw a bar of shadow across his face which gave almost the impression of a mask."
- The Shadow on the Glass - On his clothing and mask: "Mr Quin sat down. The red‐shaded lamp threw a broad band of coloured light over the checked pattern of his overcoat, and left his face in shadow almost as though he wore a mask."
- The Sign in the Sky - "[The table] was already occupied by a tall dark man who sat with his face in shadow, and with a play of colour from a stained window turning his sober garb into a kind of riotous motley."
- The Soul of the Croupier - "Mr Quin smiled, and a stained glass panel behind him invested him for just a moment in a motley garment of coloured light..."
- The Bird with the Broken Wing - Mabelle Annesley says, "I was out in the woods late this afternoon, and I met a man - such a strange sort of man - tall and dark, like a lost soul. The sun was setting, and the light of it through the trees made him look like a kind of Harlequin."
There are strong hints that he is a spirit rather than human. In one story, The Bird with the Broken Wing, Mr. Quinn contacts Mr. Satterthwaite by ouji board - which doesn't seem to surprise the latter at all.
In her autobiography, Agatha Christie listed the Harley Quin stories as her favourites and describes Harley Quin as "a friend of lovers and connected with death". The collection The Mysterious Mr. Quin is dedicated by the author "To Harlequin the invisible" which makes it unique as no other Christie book is dedicated to a character.
Christie also refers to the Harlequin character in the Masque from Italy sequence of poems in her 1925 collection The Road of Dreams (reprinted in 1973 in Poems) and in her first-ever published magazine short story The Affair at the Victory Ball (1923), published in book form in the US collection The Under Dog and Other Stories (1951) and in the UK in Poirot's Early Cases (1974).
The 1928 silent movie The Passing of Mr. Quinn, which starred Stewart Rome, was loosely based on the short story The Coming of Mr Quin and was co-adapted by the film's director Leslie S. Hiscott who went on to direct Alibi starring Austin Trevor as Hercule Poirot. The character called Mr Quin in the movie is essentially a completely different person.