Xon

Xon 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 Xon to be betrothed to the half-Romulan Saavik. (Pon Farr)

In 2285, 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. (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.

Background Information
Lieutenant Xon was written to replace Spock 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, Xon was rewritten into Commander Sonak. The character of Data on Star Trek: The Next Generation was loosely based on the Phase II idea of Xon being a Vulcan attempting to emulate human behavior.