You're asking a lot in this question, and I'm not sure if I'm clear what you're asking. I'll do my best to get your started with your answer.
I'm not sure which characters you need to compare and contrast, but I would choose the main character, Holden, and his roommate, Stradlater. Both are young men at prepatory schools, possibly from well-to-do families. This is most likely where their similarities end. Stralater is good-looking, athletically inclined and seems popular with women. He's what we would call today a "player" or a "jock." He seems to think he can take advantage of people because people like him. For instance, he almost assumes Holden will write his essay and let him borrow the coat.
In contrast, Holden is not as good-looking (gray hair, skinny, tall), is not athletic (as we can see when he attempts to beat up Stradlater) and is rather cynical and anti social. He prefers to spend time away from people--everyone is at the football game, but him, and he doesn't apply himself at school. He is what would we stereotypically call a "loner" or a "nerd."
The reader receives characterization about Holden through Holden's perspective. Holden admits that he is a liar, so the reader must read between the lines to figure out what Holden is telling the truth about, and what he is lying about. While Holden may rave about his looks and how popular he is and how other people don't "get him," an astute reader will see right through this.
The reader receives characterization about Stradlater through Holden, and because Holden dislikes guys like Stradlater (possibly out of jealousy) many of the characteristics Holden lists are negative--especially his obsession with Stralater being a secret snob and giving girls the time. However, being able to look beyond what Holden say, the reader is presented with a more accurate picture of Stradlater.