A being who was created by a transporter accident that merged Voyager crew members Tuvok and Neelix at the molecular level. Genetically a combination of Vulcan and Talaxian, the resulting individual possessed the memories and knowledge of both Tuvok and Neelix, but had a single consciousness and an identity distinct from either of his progenitors, and therefore named himself "Tuvix."
Tuvix lived for over two weeks on Voyager in 2372 and acclimated himself into the crew very well. When the ship's Doctor discovered a means of separating the combined lifeform into its original forms, Tuvix objected on the grounds that doing so would mean the end of his life as Tuvix. Nevertheless, after long and difficult deliberation, Captain Janeway ordered Tuvix to undergo the separation on the grounds that doing so would restore two other individuals to life. When the Doctor refused to perform the procedure, Janeway did it herself, successfully bringing back Neelix and Tuvok, but not without a crisis of conscience.