He is a large, muscular man with a mustache seen wearing a white sleeveless shirt, brown boots, wristbands and waist sash, and black shorts. Presumably he has done fairly well in his job as a strongman in the circus sideshow, as he appears to be out of debt and have enough money to lend to Harry.
He seems to have a short temper. However, given Harry's reputation of being a charlatan and a welcher, Luke's attitude with Harry may be somewhat justified. He has become sick and tired of Harry's excuses not to repay the money he loaned him, and is now threatening force.