Like James Bond and Han Solo, James Tiberius Kirk has always had main character energy. Nobody would have called it that back in the 1960s, of course, but the man who shares a name with a Roman emperor (those guys weren't shrinking violets either) tends to be the biggest presence on any starship bridge.
While later "Treks" have prioritized the ensemble over the individual, the original Enterprise was fueled by star power, with Kirk more often than not the hero. Six decades since they first boldly went, he and Mr Spock remain the most famous names in Starfleet.
