We are led to believe that Nick Carraway is an objective, empathetic individual who tries to reserve judgment on others. He is young but ready to step into a profession and has already entered waters unfamiliar to him. This makes him bold, in a limited way.
Nick is also well-to-do, but not wealthy.
He is nostalgic and poetic on the inside, but a bit stiff and rueful on the outside. Nick is a blank slate to others, because he doesn't tell people what he is thinking. Instead, Nick plays along with whatever is expected of him.
This first impression fades over the course of the novel as we see Nick engage in judgment and question his own actions and the actions of others.