Ser Marlon of Tumblestone is an unseen character in Game of Thrones. He is a knight, and a patron of Littlefinger's brothels, where he is said to prefer amputees. His name is as reference to the Tumblestone, one of the tributaries of the Trident.
In the books
In the A Song of Ice and Fire novels, there's no mention of a character named Marlon of Tumblestone.