||Commander Spock, played by Leonard Nimoy
Spock is second in command with science officer duties. He served with the other previous captains of the Enterprise. He became a captain of The Enterprise after Captain Willard Decker left, and sacrificed his life to save his crew. He gave his thoughts and memories to McCoy. He came back to life along with the terraforming Project Genesis, and both of them spent time on Vulcan recovering. He saved Kirk's and McCoy's lives again after the two had been arrested.
Spock is a half Vulcan with a Human mother and Vulcan father, but he looks more like a Vulcan and his physiology is primarily Vulcan. He controls his emotions by studying logic and philosophy just like other Vulcans, and like other Vulcans, he is able to live a very long time.
Many decades later, he became an ambassador, and continues with attempts to reunify the Vulcans and Romulans.