I originally attempted to stretch this into something novel-length but found myself struggling with it. I decided instead to compress it and just get the concept out there.
The man lowered himself into the couch's softened angles, leaving Patrick holding his coat and hat. With the hat removed, the light of the room could diffuse itself across the otherwise cold lines of the man's face. Although he appeared to be only Patrick's age, a grayness lingered in his pallor as if his skin were smudged with dusty newsprint. The pudge around his middle smiled in the crease of his shirt.
"One year, about, since we last saw each other?" he said.
Patrick hummed an affirmative and lowered himself into a seat across from the shadow man reclining on his couch. He slumped back into the seat's soft paunch with a quick wheeze.
His shadow smiled. "You do seem to be doing well," it said.
Patrick said nothing. He dipped his hand into the wooly steam expelled from the humidifier at his chair side. A water-dimpled paper dripped from his writing desk and his shadow's smile closed over the bright sparks of its teeth.
Although the two wore identical features, the shadow filled his out in pockets of self-fulfilled muscle. Patrick, slumped in his steam, appeared as a wraith. Each cough crumpling his paper sack lungs warped his body with the exertion. He excused himself for a moment as he hacked into a wad of Kleenex. It seemed to his shadow that Patrick was thinning out more each time he visited. Each year compressed him, making his breath come shorter and stickier. Eventually he might be flattened so much as to become nothing but a shadow of himself, a line on the ground.
Every child imagines being able to outrun his shadow; to jump high enough that the bonds of his feet may not reseal once he touched ground again. Only few have succeeded. As soon as Patrick's coughing fit passed, the shadow repeated his offer as he had each year.
"Everybody needs a shadow," the gray man said as he always had. "Why don't you be mine?"