Harry Tressler is a fictional character from the BBC medical drama Holby City, played by Jules Knight. He first appeared in the fifteenth series episode "The More Deceived", broadcast on 14 May 2013. Harry was introduced as a CT1, later progressing to CT3. Knight's casting was announced in January 2013. He auditioned four times, with the producers taking two and a half months to choose him for the part. To help prepare for the role, Knight shadowed a surgeon and watched a laparotomy. In February 2015, it was announced that Knight had quit the show in order to concentrate on the release of his first solo record. Harry departed during the seventeenth series on 14 April 2015.

Mary-Claire Carter is a fictional character from the BBC medical drama Holby City, played by actress Niamh McGrady. She first appeared in the eleventh series episode "Spin", broadcast on 22 September 2009. The character was introduced by Tony McHale. Between her arrival and January 2013, Mary Claire appeared on a recurring basis, sometimes with months between appearances, as an agency nurse on Keller and AAU wards. McGrady was promoted to a regular cast member in early 2013 by newly appointed executive producer, Oliver Kent - alongside the introductions of three characters. Her first appearance, as a regular cast member, aired on 23 April when Mary-Claire received a staff nurse job on the AAU ward. She is characterised as a blunt and easily distracted character, yet a brilliant nurse. Her strong point is not diplomacy, but scratch beneath the surface and you'll find a heart of gold. McGrady has described her as "cheeky, flirtatious, and trouble."