Steth was a test pilot of a coaxial warp ship whose identity was used by a body-swapping alien to take over Tom Paris's life.
Steth was an alien test pilot flying a ship with a working coaxial warp drive, a technology Voyager's crew had thought purely theoretical. When his malfunctioning ship encountered Voyager, the restless Tom Paris eagerly helped with repairs, and the two quickly became friends. Dan Butler played the pilot Voyager met.
In Vis à Vis, the Steth Voyager knew was revealed to be a body-swapping alien who had stolen the real Steth's identity. He exchanged bodies with Paris and stayed aboard Voyager while sending Paris away in his old form. Paris found his way back, and the crew undid the swap after the alien moved on to Janeway.