Nick is the main character of Ernest Hemingway's short story "The End of Something." We don't get a last name, we don't get an age, or any sort of back-story on Nick really, just his name, that he's a guy, and that he breaks up with his girlfriend, and has a friend named Bill that shows up at the end. That is really all of the information that we get about him. This story, written in the Modernist style, gives only bare-bones minimums, and lets the reader fill in all of the gaps however they want to. Modernism has several features of its writing that are all featured in this story: characters that are alienated and have trouble communicating, little detail and thought description, and the heavy use of symbolism to represent characters and their dilemmas.
Nick and Marjorie's deteriorating relationship is symbolized by the crumbling ruins of the mill; we don't get descriptions of their thoughts, just their actions, and they are obviously struggling to connect. With only this information, the reader is left to infer, or guess, what is going on behind the scenes. We can infer that Nick is rather young; after all, he's going fishing with a girlfriend, and that seems like an activity that a younger man would do. He isn't married or committed, so he is probably a teenager or young adult. He has a close friendship (or more) with his friend Bill, because Bill obviously knew he was going to break up with Marjorie, and Nick had been dating Marjorie for quite some time, long enough for them to have memories of the mill operating.
I hope that those thoughts helped; good luck!