Okay, so is this guy our author or some character? Can't he just decide? Well, no actually—because he's totally both. And that fact is actually pretty important. More enjoyed playing with the idea that Utopia is a real place and that everything in his book actually happened. What better way to make this fiction seem even more true than by making himself a character who is also the narrator? That way, he can attest (or he can pretend to attest) that he actually met this guy Hythloday and everything he's writing down is the 100% honest truth. Nifty, right?
But why? While More clearly thinks this whole fact/fiction game is kind of hilarious, he also takes the stakes of what Utopia is about philosophically and morally very seriously. By placing himself in the text as a narrator and swearing up and down that it actually happened, he's placing the conversation and the topics it raises in the real, immediate present of Europe. Essentially, it's a way of saying that even though everyone will actually understand that there is no such place as Utopia and no such adventurer as Hythloday, the issues at hand aren't just fun fantasies, but of real importance to Europe and to its governments.
When Utopia was written, most people in England would have known who Thomas More was. He was an intimate advisor to King Henry VIII—check out "Why Should I Care?" for more details on him—and was something of a scholar and translator too. So, let's just say, he's something of an authority on issues of politics, scholarship, and philosophy. His actual fame partly explains the lack of info we get from the book itself. All we're told is that he works for the king of England and, like Hythloday and Giles, seems to have a fascination both with issues of philosophy and stories of travel.
He also (like Giles) functions as a voice of skepticism in response to both Hythloday's extreme pessimism toward the European world and to his extreme enthusiasm for the Utopian way of life. More primarily takes issue with Hythloday's unswerving belief that a philosophically-minded person can't make a difference in a political world. At one point, he calls Hythloday out for complaining about how unphilosophical politics are without doing anything about it:
"No wonder we are so far from happiness when philosophers do not condescend even to assist kings with their counsel." (1.28)
Ouch. While he (and Giles) are far less vocal in Book 2, the book ends with More sharing his doubt that Utopia really is as spectacular as Hythloday believes.