Monsieur du Plessis was a human teacher in France on Earth in the 24th century.
During Jean-Luc Picard's childhood, du Plessis was responsible for teaching Picard Latin and history when he was in school. Later, Picard remembered du Plessis's lessons on the Roman Empire when he was dealing with the Magna Romans of the USS Centurion. Picard thought of du Plessis as "dictatorial". (TNG novel: The Captains' Honor)