Seth Costello is a fictional character from the British television soap opera Hollyoaks, played by Miles Higson. He made his first appearance during the episode broadcast on 20 July 2010. His first major storyline was in 2011 when he was the centre of a steroids storyline. He also became involved in the storyline where his father Carl Costello began an affair with Mercedes McQueen. 2020 saw the return of Seth, when he blackmailed the McQueen family with grandfather Silas Blissett. He departed again on 2 February 2021.

Carl Costello is a fictional character from the Channel 4 soap opera Hollyoaks, played by Paul Opacic. The character was introduced by executive producer Paul Marquess and he debuted on screen during the episode broadcast on 19 July 2010. Carl arrived in the village as the new owner of The Dog in the Pond and the father of established character Jem Costello. Carl departed from Hollyoaks on 1 December 2011 with his son Jason Costello. Opacic later reprised his role and returned to Hollyoaks on 26 September 2012 and left again on the 5 October 2012. Opacic reprised his role once more in January 2013, which coincided with the exit of Mitzeee Minniver. He returned on 25 January 2013 and departed again on 15 February 2013. Opacic returned again for a guest appearance on 10 July 2018, which ended in his character being murdered by an unknown assailant on 11 July of that year. He reappeared via a flashback on 16 November 2018, which revealed Breda McQueen to be his killer.