Mademoiselle de Neuf ("Miss of Nine") was the identity of Seven of Nine in the French Resistance holodeck simulation created by the Hirogen. A munitions expert posing as a piano singer at the Le Coeur de Lion, de Neuf was a member of the French Resistance movement against Nazi Germany's occupation of France during World War II.
Mlle. de Neuf had originally worked with the underground and was characterized as head strong by the leader of the Saint Claire resistance team, Katrine. She and Katrine had often disagreed over the best courses of actions to take in the fight against the occupying forces. de Neuf wanted to directly attack the German forces, while Katrine insisted on maintaining the team's undercover identities. She was suspected of being a Nazi spy by one of her colleagues after Katrine's old friend and bartender first raised the possibility based on Mlle. de Neuf's regular habit of challenging Katrine's orders and penchant towards recklessness.
Through the use of an implanted neural interface, the crew of the USS Voyager believed that they were the characters in the simulation. Seven of Nine's real identity was restored by The Doctor; however, she continued to play along with the simulation, unbeknownst to the Hirogen. After disabling the crew's neural interfaces, Seven of Nine and Captain Janeway were able to stage a counterstrike with support from the holographic characters. Seven later modified explosives to emit a photonic burst which disabled holographic activity on part of the ship, a crucial event which allowed Captain Janeway to defeat the leader of the Hirogen and return control of the ship to the Starfleet crew. (VOY: "The Killing Game, Part II")