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")