A moustached, womanising helmsman in the Doctor's holonovel Photons Be Free, an obvious caricature of Tom Paris that prompts Paris to write a parody in return.
Lieutenant Marseilles is a character in Photons Be Free, the holonovel the Doctor writes in the Voyager episode Author, Author. He serves on the fictional USS Vortex and differs from Tom Paris chiefly in having a moustache.
In the story Marseilles is a married officer who uses sickbay for assignations and bullies the ship's holographic doctor into keeping quiet about it. Paris, playing through the programme, is offended on his own and his wife's behalf.
He answers by rewriting the holonovel so that the Doctor experiences a version in which the hologram is a leering, comb-over wearing lecher. The joke helps the Doctor see how his portraits feel to the people behind them.