Phoebe is his kid-sister. An adorable, stubborn young thing who makes it clear she would do anything for her older brother whom she admires and respects despite his flaws. He visits her in the middle of the night just to "chew the fat" with her. He buys her a record in New York just because he thinks she'd like it (although when he's uber drunk he drops and smashes it). He gives it to her anyway and she keeps it. It's the thought that counts.
D.B is Holden's older brother who is a writer. According to Holden he writes the most amazing stories, but he sold himself to Hollywood and now he writes movies. D.B. doesn't really play that much of a part, but a lot of Holden's opinions on life and people obviously stemmed from D.B.'s influence.
Allie. How do I even begin to describe how the relationship between Allie and Holden made me feel?! Ok. I am just going to say this up front; I was on page 36 thinking that although Holden was an interesting person, there was little depth to him. Depth didn't seem to be his thing, but then his roommate asks him to write a composition for him, so Holden writes about Allie's old baseball mitt (a left-handed mitt covered in poetry). Then using his classic train of thought he starts talking more about Allie and about how much we would like him, if he were still alive...
"...He was a nice kid, though. He used to laugh so hard at something he thought of at the dinner table that he just about fell off his chair. I was only thirteen, and they were going to have me psychoanalyzed and all, because I broke all the windows in the garage. I don't blame them. I really don't. I slept in the garage the night he died, and I broke all the goddam windows with my fist, just for the hell of it.... It was a very stupid thing to do, I'll admit, but I hardly didn't even know I was doing it, and you didn't know Allie."
Any part that included stories of Allie or interactions with Phoebe were my favourite.