Why is Holden important in The Catcher in The Rye by J.D. Salinger?

Holden Caufield is significant in J.D. Salinger's The Catcher in the Rye because he serves as its protagonist. This novel explores Holden's story of running away to New York City after being expelled from the prestigious Pencey Preparatory boarding school in Agerstown, Pennsylvania; it is all told in a first-person, stream-of-consciousness narration by Holden himself.

Holden Caulfield, who is staying in a sanatorium, opens the book by speaking directly about his willingness to discuss what happened in those few days in December of 1949 after he fled Pencey Prep:

...I'm not going to tell you my whole goddam autobiography or anything. I'll just tell you about this madman stuff that happened to me around last Christmas just before I got pretty run-down and had to come out here and take it easy.

Holden is sixteen-years-old, lonely, and in search of an identity that will fit into the strange world around him that is full of "phonies." He leaves Pencey Prep because he has been failing four subjects due to his refusal to apply himself and he is utterly irresponsible (having left the fencing team's equipment on the subway, which forces them to cancel their meet). Before leaving the school for good, Holden visits his history teacher, Mr. Spencer, at his house and later gets into a fight with his roommate, Stradlater.

All too eager to stave off delivering the bad news of his expulsion to his parents, Holden hops on a train to New York City and checks into the Edmont Hotel. There, his unhappy adventures and time of introspection begin, as he dances with three female tourists twice his age, visits a nightclub in Greenwich Village, and hires a prostitute to come to his hotel room—all in search of his sexual identity. 

After Holden fails to have sex with the prostitute (which subsequently results in the woman using her pimp to bully Holden out of more money), Holden finds himself feeling even more alienated than before. He sees a play with an old friend, Sally Hayes, and takes her ice skating at Rockefeller Center. Holden asks Sally to run away with him, but she refuses, and their date ends poorly. Holden gets drunk, goes in search of some ducks at Central Park, and, in the process, breaks the record he purchased for his younger sister, Phoebe.

Defeated, Holden sneaks into his parents' apartment to see Phoebe and speaks to her of his private dream of serving as the guardian (the "catcher") of children playing in a field of rye on a cliff. Holden then leaves the apartment to visit another former teacher, Mr. Antolini. Mr. Antolini ultimately disappoints Holden by failing to understand Holden's perspective on life. The next day, Holden takes Phoebe to the Central Park Zoo and finds some joy in watching her ride the carousel.

Ultimately, the institutionalized Holden re-telling this story rushes to wrap it up, suggesting he fell ill, which led him to be in a sanatorium. He plans to return to school in the fall, although he is irritated by everyone's questions about whether he plans to apply himself. He closes the novel by urging the reader, "Don't ever tell anybody anything. If you do, you start missing everybody."