Unnamed man is the brother of George, mentioned by George Costanza.
Not much is known about him, only that he is the brother of George Costanza and the other son of Frank and Estelle Costanza. In "The Suicide" episode (Season 3), George mentions that he has a brother who impregnated a woman named Pauline. In "The Parking Space" (Season 3), George mentions that his brother refuses to pay for parking much like the rest of the Costanzas.
In the episode "The Parking Space" George is in the car with elaine and mentions his brother.
George: I don't know, I just can't. Nobody in my family can pay for parking, it's a sickness. My father never paid for parking; my mother, my brother, nobody. We can't do it.