We know Nick is from a comfortable, well established (but not wealthy) family in the Midwest that has taught him the value of work. He served in World War I. After returning home, he was restless and went to New York to learn the bond business. His family supported him financially for one year in order for him to go, but they expected him to support himself after that.
We know that while in the East something very significant happened to Nick in relation to someone extraordinary he knew there named Gatsby. We can infer that Nick is a sensitive person because he still remembers whatever it was that happened, and it changed him in a significant way. Before going to the East, Nick had been a very non-judgmental person; he returns, however, having developed a very strong sense of moral (and immoral) behavior, and he condemns immoral behavior. Nick's first encounter with the Buchanans in East Egg shows that his Midwestern values are going to clash with the sophistication he finds in the East. Nick seems to be a trustworthy young man. His family trusted him with money, his college friends trusted him with their confidences, and nothing in the first chapter suggests that will not be a reliable narrator.