The un-named Romulan Commander from the episode "Balance of Terror" - he was portrayed by Mark Lenard, who later went on to portray Spock's father, Sarek.
Wikia Entertainment wrote:
Around stardate 1709, a Romulan commander piloted a Bird-of-Prey on a mission testing a cloaking device, and a new energy weapon that destroyed Federation outposts along the Romulan Neutral Zone. He was an atypical Romulan commander who questioned the ethics of his mission and was wary of war. He believed that enough lives had been sacrificed for the Romulan quest for conquest.
He and James T. Kirk would engage in a deadly game of cat and mouse. At first he tried to return to Romulus to deliver the information on the new weapon, but did turn and fight when the Enterprise intercepted him. When his ship was disabled by the USS Enterprise he destroyed it, rather than be captured. He told Kirk that he admired him and that they were alike, and could have been friends in another reality.