Master of the House's mistress

A century before “the coming of the Elevator”, the then-current Master of the House of the Strange and Wonderful House kept a mistress. After a parlour maid discovered the true nature of the Airing Cupboard, he installed a life-sized portrait of her over the door. This outraged the Master of the House's wife, and it was later said that she instantly moved back to the continent, where she died of jealously soon after. A century later, when the disturbances caused by the Elevator's appearance caused the door to the Cupboard to be rediscovered, a visitor entered the Cupboard only to be confronted by a ghostly figure in a Victorian nightdress who referred to him as “darling” and pulled him into a deadly hug. (PROSE: Our Strange and Wonderful House: The Airing Cupboard)

Behind the scenes

There are many possible interpretations of the end of the The Airing Cupboard, from the ghostly figure being the mistress, to being what's left of the wife whom the Master locked into the Cupboard instead of her moving to the continent as was rumoured, to being unrelated to either.