He was the half-brother of Marcel Desailly, who later followed him to the FC Nantes Atlantique. Adonkor made his debut for the 'Canaries' on 15 April 1981 in a 2–2 draw against Metz, but only established himself in Jean-Claude Suaudeau's first team in the 1982–83 season, which his club finished as French champions, nearly adding the Coupe de France to their honours list – a game in which Adonkor was sent off. The 1982–83 championship was to be his only major honour. By trade a strong, hard-tackling midfielder who could also play in attack, Adonkor had become a 'rock' (in Suaudeau's words) in the heart of Nantes's defence when his career was cut brutally short. Driving back from Saint Nazaire to Nantes with his teammates Jean-Michel Labejof (who died) and Sidi Kabahe (who survived, but never played again), he was the victim of a fatal car crash. Adonkor was then widely tipped to become a French international. The news of Adonkor's death was passed on to Marcel Desailly by his close friend and teammate Didier Deschamps. Adonkor was Desailly's role model. Adonkor was an impressive centre back who was close to Marcel, when the news had broke Marcel was deeply saddened however was determined to make the rest of his career a tribute to his late half brother. Adonkor was hugely successful at FC Nantes academy and was tipped to be a regular starter in the team. However this was not to be with the events that had occurred in November 1984. Seth has always been remembered by FC Nantes fans as the one with a lot of promise but was taken too early.