Nick is the moral center of the novel, and he sees how hollow the lives of the other characters are. Though attracted to Jordan, he recognizes her inability to make a commitment to anyone. He doesn't have the romanticism of Gatsby and sees the lives of the people on West Egg for what it is, false and unmeaningful. Nick inherited his code of conduct from his father, so he tells the story without judging the other people. Whenever he feels the urge to criticize, his father said to remember "that all the people in this world haven't had the advantages that you've had". Nick's solid background in the Midwest allows him to see life as it really is. Nick feels for Gatsby in the end, telling him that he's better than the "whole rotten bunch put together". Nick is able to distinguish between good and bad, appearances and reality, and truth from lies. He's a reliable narrator, but at the same time, he's a sincere narrator.
At the beginning of The Great Gatsby, Nick tells us about how he, because of his father's advice, tends to reserve judgment about people. This immediately establishes him as a narrator who tells us the events without a great deal of judgment, letting us do our own assessing of the characters. By having Nick tell us the story , we get to see through the eyes of one of the participants in the events which draws the reader into and closer to the story. Also, since Nick is in a social class that falls between Gatsby and the Buchanans, we are given a more impartial glimpse into both worlds. Finally, Nick lets the reader know from the beginning that he somewhat understood the very complicated character of Jay Gatsby when no one else did.
It is very interesting that Nick is the narrator who comes to sympathize with Gatsby's plight. Daisy is his second cousin, and he went to school with Tom, yet, clearly he sides with Gatsby. Nick was surprised that he became so fascinated with a man he normally would never acquaint himself with. Nick recognizes the fallibility of Jordan and others,and claims to being non-judgemental therefore, in the end he is a fairly reliable narrator.
In my opinion Fitzgerald used Nick as an objective person in the mist of the story, although there were times when he became subjective it is used as the underlying theme of the novel that we are all human. However Nick does bring to the story a since of morality. It is almost as if he is the fly on the wall but a very interactive fly. If the story was from other perspectives then it would be biased towards a character.