From Wikipedia, the free encyclopedia. Marcello Thedford is an American film and television actor. His most noteworthy roles have been as Kelvin "Buffalo" James on the ESPN dramatic series Playmakers, Semi in Employee of the Month, and in eight episodes of ER as Leon, Dr. Greg Pratt's mentally challenged brother. Thedford's acting career began in improvisational theater in New York City. He studied there at the Third World Theater at Black Drama Productions. His first significant role came in ...