Like all of the Mego figures, his body is copyrighted by Mego, but his head belongs to Paramount, leaving very little for Leonard Nimoy.
Kirk relied on Spock a great deal. Spock was almost always ready with any needed information, and his ability to remain objective and logical in crisis was often critically important, especially if Kirk himself was compromised. Spock was intensely loyal to Kirk, and seemed to have a sense that Kirk's human emotions sometimes gave him a perspective that justified a course of action Spock wouldn't have followed.
For all of his formal education, Spock had one area of ongoing study: human nature. This was complicated by the fact that Spock was himself half human, and his human emotions were constantly flaring up to disrupt his pure Vulcan logic.
I think that what I like most about Spock is his strong mental discipline. My favorite example of this is in “Operation -- Annihilate!”, in which he overcomes wracking pain by regarding it as an emotional response and suppressing it.