Marseilles was a hologram, based on Tom Paris, that was a character in a holonovel written by The Doctor entitled Photons Be Free.
He was the helmsman of the USS Vortex. In "Chapter 1: A Healer is Born", Marseilles was brought to sickbay by Commander Katanay with a slight concussion, having been injured during the incident with the Caretaker. When the person playing the EMH refused to treat Marseilles first, wanting to see to a more seriously injured patient, Captain Jenkins would kill the other patient so that Marseilles would be treated. He was a womanizer, although he was married to Torrey, a character based on Paris' wife B'Elanna Torres. While playing The Doctor's role, Torres caught him kissing a female crewman in sickbay. The Doctor described the character as being self-indulgent and immature when compared to the real Paris. (VOY: "Author, Author")
Like Paris, Marseilles was portrayed by Robert Duncan McNeill.
The name appears to have been chosen as a joke, since Marseilles is a city in France, as is Paris.
From the Star Trek Encyclopedia, 4th ed., vol. 2, p. 22, "Photons Be Free was presumably published some time after the conclusion of the episode, albeit in revised form. We suspect that the final version of the character was less like Paris, and may even have had a different name."