Variously described as a genetic engineer "amongst other things" and an expert on working in highly radioactive areas, Justin was head of a scientific warfare team sent to Bucol-2 to work on a top secret experiment in genetic engineering, creating intelligent, radiation-proof creatures to act as shock troops in contaminated zones (normal troop loss was 60% in such areas, due to the radiation). When the other scientists left towards the end of the Intergalactic War (and were shot down by an enemy gunship on their departure), he elected to remain, to continue with the experiments and look after the animals he had helped to create. He was still there when Dayna arrived, intending to conscript him and his skills for Avon's struggle against the Federation. Avon thought he might be able to engineer an antidote to pylene-50. Dayna was unaware that Justin had worked for the Federation, and Avon was presumably likewise misinformed - no information was given as to how Justin's location was traced (by Orac?) without any indication of who had sent him to Bucol-2 in the first place.
Justin was a friend of Hal Mellanby, and had visited Sarran to tutor Dayna. Since the research on Bucol-2 had started six years before the War, and Dayna had been on Sarran for twenty years when the war started, leaving Earth as a small child, she could not have been older than her mid-teens when Justin taught her. That doesn't appear to have stopped Justin taking an interest in her. On Bucol-2 he persuaded Dayna to help him with his work, which was now directed towards using the animals to rebuild contaminated war zones, ignoring the likelihood of people not being able to live in such places for some while afterwards. After being taken by Servalan, Dayna was induced to hate Justin and she arranged for his capture. On Servalan's ship he surreptitiously informed Avon of his and Dayna's whereabouts, but was shot by Servalan whilst attempting to escape. His body was then dumped out of the ship as it took off.