Sivok was a male Vulcan who, as of 2269, was a Captain in Starfleet, and commanding officer of the science vessel USS T'Saura. He was characterized as a man confidence in his crew and their abilities, and during a mission to the Delta Triangle region in 2269, had initially refused offers of assistance from the Corps of Engineers crew aboard the USS Lovell. However, when ordered to work with the Lovell team by Admiral Byron Komack, he did so without any issue.
Sivok was killed, along with his entire crew, when the T'Saura was caught in the center of a contracting field of mines, laid by a Gorn vessel to blockade the Delta Triangle, and his ship destroyed. (SCE eBook: Where Time Stands Still)