The term "H" in "line of duty" refers to a character in the BBC TV drama series "Line of Duty," who is a corrupt police officer operating within the police force. The identity of "H," the mysterious nemesis in the show, was revealed in series six as Detective Superintendent Ian Buckells, played by Nigel Boyle. The character "H" was involved in corrupt activities while on duty as a police officer, making him the key figure "in line of duty" related to corruption and wrongdoing within the police.

If the question was about who was "in line of duty" as in who served in duty roles generally, typically in contexts like police, military, or similar service sectors, the TV show "Line of Duty" centers around various police officers like Steve Arnott, Kate Fleming, and Ted Hastings, who are main characters serving "in line of duty" as anti-corruption officers.

To clarify, "H" is a fictional character representing corruption "in line of duty" within the police force setting of "Line of Duty" series, rather than a general question about duty service in real life. Please specify if a different context is intended.