Holden’s relationship with phoebe in chapter 21 is coles holden cares alot about phoebe. Holden always tries to protect her. Holden is so comfortable around her because she not hard on him, she understands him, and she always there for him.
Holden has been told many times to grow up, like mr.spencer told him to stradlater told him and carl luce etc. holden isn't legally a adult but he thinks he has adult features such as gray hair, and he's tall. With Holden and Phoebe Holden’s the child and Phoebe is the adult. The reason why Phoebe is so mad at him for getting kicked out of school is that this happens a lot and she had faith in him.
When Holden talks to others he isn’t himself he has to put up a front to keep them there.