Detective Sergeant (later Inspector) Gavin Troy (Daniel Casey) (series 1-6, series 7, series 11) DCI Barnaby's first assistant. Troy is bright and ambitious. Early in the series, Troy is also known for his careless driving, causing a number of near-accidents (which does not stop at all, even in his final/guest appearance).

Troy is also prejudiced towards gay people (in the very first episode, The Killings at Badger's Drift, he shows his discomfort by using a slur against the gay undertaker & in the episode Blue Herrrings, he uses a slur against a character who he perceives as being gay) and the elderly, referring to them as "wrinklies".

In Market For Murder and Destroying Angel Troy provides valuable insight, finding out how the killers committed the murders, so oddly it was not Barnaby who solved those cases. In Painted In Blood, Barnaby is forbidden to take part in the investigation, but Troy supplies him with information. Later on, Barnaby is taken hostage in a bank robbery and might have died until Troy sneaked in and attacked the robber, yet at the same Troy would himself have been killed had Barnaby not stepped in, but together they overpower the robber. Troy's relationship with Barnaby is warm, and the two make a formidable pair.

Troy was promoted to Inspector and transferred in the first episode of the seventh series, called The Green Man. Troy makes one re-appearance in the first episode of Series 11, Blood Wedding, to attend the wedding of Tom's daughter, Cully Barnaby (whom he once secretly kissed, which temporarily strained his relationship with Barnaby), where he met his second successor, Sgt Jones.