Xon was a male Vulcan who was a member of Starfleet Intelligence. Xon was born on Vulcan in the early 2260s and was betrothed by the 2270s. After his original bond-mate died in a tragic accident, Ambassador Sarek arranged for Xon to be betrothed to the half-Romulan Saavik. (TOS - Saavik's Story comic: "Pon Far!")
Later that year, on stardate 8185.35, Xon was sent by the Federation on a secret mission to the galactic barrier. Posing as a Romulan, he investigated the Romulan plot to duplicate Gary Mitchell's powers, before being discovered and interrogated. He was rescued by Saavik and the crew of the USS Enterprise. (TOS - Saavik's Story comic: "Blood Fever")
- It is unknown what happened to Xon after 2285 and his pon farr with Saavik. Saavik would later marry Spock in 2329, so it is possible that he either divorced her, or died, or even never formally married her for some reason.
Lieutenant Xon was written to replace Spock as science officer in the planned series Star Trek: Phase II, where he was to be played by David Gautreaux. By the time of Star Trek: The Motion Picture, Spock's replacement was rewritten as Commander Sonak, in case the Vulcan character extensively researched by Gautreaux might yet have a place in future productions. According to a Starfleet Academy personnel manifest in the background of the area outside of the Kobayashi Maru scenario simulator, a Captain Xon had an office located at #213 (Star Trek II: The Wrath of Khan). The character of Data on Star Trek: The Next Generation was loosely based on the Phase II idea of Xon being a Vulcan emulating human behavior.