Corky previously appeared in parts of the 'Documentary' storyline, my confusingly high-concept idea that depicted the events of the strip as being a television show, with the wider context being the inverse: the bit about it being a TV show was fictional, while the events of the 'episodes' were canon and in-continuity.
Katie Corcoran was the original tenant of 21 Nixon Street and took in Halifax as a lodger after he returned from Russia, immediately after parting ways with Holly. We originally saw Corky in Shit Happens: Pilot, Sofa, So Good, Death Comes To Gothereswick, and the decidedly un-PC named A Girl, A Gay, Two Drunks, and a Tranny. We originally saw the polaroid of Corky that Halifax carries around in his wallet in the storyline Whatever Happened To Katie Corcoran?, which contains possibly the only instance in human history of the phrase 'Genital Design Awards'.