Sirok was a male Vulcan who was an older distant cousin of Spock and by 2248 was a student at the Vulcan Science Academy. By appearance, Sirok was very much like his cousin Spock with both possessing dark hair, pointed ears, sharkply slanted eyebrows. Notable differences included Sirok being paler, thinner, taller and more Vulcan.
In 2248, Spock met with him for the first time and Sirok expressed the fact that many of the Vulcan masters at the Academy were skeptical of his abilities. This was because of Spock's Human heritage and Sirok expected his younger cousin to fail during his time at the Academy. He was later attacked at the Vulcan Science Academy by dissident Marathans who believed him to be Spock which led to Sirok suffering serious injuries that resulted in him being sent to a Healing Center. (TOS novel: Crisis on Vulcan)