Phoebe is probably whom I also consider to be the least phony character. I think that Holden's reaction to her is what helps convince me that she is the most forthright and most sincere. Throughout the novel, Holden recoils at the phoniness and insincerity of others. Yet, he actually displays some sentiment of affection, compassion, and love with Phoebe. She represents that which is good and pure for Holden. It is almost as if the notion of innocence and childhood go together. With the shrinking social and emotional circle of Holden, he is still willing to open up and communicate with Phoebe, who speaks her mind to Holden and represents a spirit of pure hope that might offset the phoniness around Holden and his own internal sense of misery that is a reflection of it.
In my opinion, Holden's sister Phoebe is the least phony character in this book.
The reason that I say that is that she does not seem to be trying to pretend to be something that she is not. She seems to have genuine emotions and to act based on those emotions.
I think that this is probably because she is the youngest character in the book -- the only one who is not sexually mature. I think that the book is sort of saying that sex is a lot of what makes people phony.
That brings me to my other choice for characters who aren't phony -- the nuns. But I figure that they are not enough of character to count.
Anyway, Phoebe lets people know what she's thinking. She loves Holden, but when she's mad at him she won't let him touch her, for example.