Ducos is described as short, his face scarred by smallpox. Weak and timid when physically confronted, he claims to detest ‘unnecessary violence’ but is utterly indifferent to the suffering of others. He is despised and feared by most of his colleagues, who nonetheless obey him out of fear. Ducos seems entirely motivated by his dedication to the ideals of the French Revolution, continuing to use the Republican Calendar long after it has been abollished (Sharpe's Enemy). He has no personal friends and his only indulgence is the occasional visit to prostitutes (Sharpe's Battle).
Until he first crosses paths with Richard Sharpe, his schemes were largely successful and he is well regarded by Napoleon, who sent him to Spain to resolve what Napoleon described as the "Spanish Ulcer". He is said to have bribed the Spanish commander of Badajoz to hand the keys of the garrison to the French.
He first comes in conflict with Sharpe in 1812, during Sharpe's Battle. During the events of Sharpe's Enemy, he viley insults Sharpe's wife, Teresa. Sharpe pulls his sword, but rather than harm Ducos, insults him with the destruction of his spectacles in return.
In 1813 Ducos uses Helene Leroux to set a trap that he hopes will destroy Sharpe. When he later captures and tortures Sharpe, he destroys Sharpe's prized telescope in retaliation for his own broken spectacles. Sharpe used the broken instrument to stab his torturer and escape.
In 1814, Ducos anticipates the fall of Napoleon and steals the Imperial jewels, implicating Sharpe and Frederickson for the crime, and retires to the comfort of a villa in Naples. He is once again bested by Sharpe, who escapes from custody, and captures Ducos. As he seems to do with his most implacable enemies, Sharpe did not take personal revenge, but turned Ducos over to the authorities, who sentenced him to a traitor's death. He was shot by firing squad and died in a fortress ditch. He was buried in an unmarked grave.
Ducos flees when Sharpe escapes the Fort of Santa Maria in Sharpe's Honour, and is apparently shot and wounded by a looter at film's end. He reappears in Sharpe's Siege, behind Calvet's attack on the castle held by Sharpe.
Sharpe is next set up by Ducos in Sharpe's Revenge, being charged with stealing the Emperor's jewels, which Ducos himself has made off with, down to Naples. Frederickson manages to find Ducos via his order for new spectacles. In the final confrontation, Sharpe shoots Ducos off his horse, and he is then dragged, presumably dead.
In the TV movies, Ducos was played by Féodor Atkine.