The Mysterious Mr Quin by Agatha Christie

Harley Quin is an enigma. Even his friend Mr. Satterthwaite is unable to understand how the man seems to appear and disappear almost like a trick of the light - and when he does appear it's usually in the sparkle of sunshine, or surrounded by a spectrum of coloured light pouring through a stained glass window.

Indeed, he is Harlequin. The only consistent thing about the mysterious Mr. Quin is that his presence is always a harbinger of love .. or death.