Karl Demmer

is a retired FBI agent. He, along with fellow agent Marmaduke Stone, had the first successful capture of a gamma, Clemson McCain, in 1964, which is the subject of episode 3x00,"On Faith".

He retired from the FBI after a car accident when he was forty-five. As of 1995, he and Stone live in Manassas, Virginia. He was still alive as of Christmas, 2009.