When most people think of Star Trek, they of course think of Kirk and Spock. This isn't surprising, as they were the star and the break-out character of the series, but it's easy to forget that they were two members of a triumvirate that made up the essence of the show. Ship's doctor Leonard "Bones" McCoy was the third, and just as integral in making Star Trek a joy to watch.
While Spock was all cold logic and Kirk was fiery passion, Bones was the heart of the Enterprise crew. He was the one who made the plea to do the right thing, who reminded the others of the potential cost of obeying logic or the Prime Directive. McCoy also was the true audience surrogate of the show, saying the things we yelled at the TV. The crew of the Enterprise did run into some completely ridiculous phenomena, and none of it would have worked if McCoy wasn't there to say "Of course you save their lives, you idiots!" and "How in the universe is this happening to us?"
What makes Bones so fascinating is that he clearly didn't want to be there. He wasn't a full believer in the Starfleet mission. He hated space and ships and Starfleet, but he went out there and did his job every single time, no matter what insanity he encountered. Remember "The Devil in the Dark?" Can you imagine starting out life as a doctor in your home state and then finding yourself being asked to treat this?
McCoy complained, but he did it. Because he was a doctor and that was his job. And Kirk ordered him to. McCoy's history isn't epic and tragic like Kirk's or unique like Spock's. Which is why he, despite being an actual genius, insisted over and over that he was just a country doctor.
And when I say McCoy complained, I mean he complained all the time. About everything, but mostly Spock. Just think about how much life Star Trek would have lost without Bones for Kirk to talk to or for Spock to spar with:
Spock: I'm happy the affair is over. A most annoying emotional episode.
McCoy: Smack right in the old heart. Oh, I'm sorry. [pointing to his side] In your case, it would be about here.
Spock: The fact that my internal arrangement differs from yours, Doctor, pleases me no end.
(I would literally watch Spock and McCoy trade barbs over any other duo in all of Star Trek's 50 years.)
In between insults, grumpiness, and straight-up bitching, McCoy was also responsible for delivering messages about the human condition more than anyone else on the show. Kirk had the mission, Spock his logic, but what McCoy cared about was his patients -- and that made him closer to people than he was to anything else.
One of Star Trek's primary messages is about the importance of the balance within ourselves. (It's why several episodes revolve around characters splitting into two halves.) McCoy was an integral part of that equation for TOS, just like DeForest Kelley was integral is making Bones the perfect mix of grumpy and compassionate, prickly but still so essentially humane.
Kelley may not have received the same level of fame as William Shatner or Leonard Nimoy, and Bones might not have gotten the flashiest scenes. But Dr. McCoy was still the heart of the whole show, and it would have never taken off without him. Bones may not have wanted to be in space at all, but thank goodness he was.