The Chuck Cunningham character was around for more than the pilot. He was not present often in the story line, but several times referred to (he was away at college, on a basketball scholarship, IIRC). Chuck appeared on screen occasionally in the first two years of the show. Some time after that, Chuck disappeared into the TV ether.

Chuck was played by two actors (tying into the replacement actors branch of this thread ...): Gavan O'Herlihy and Randolph Roberts.