Phoebe Josephine Caulfield is Holden's younger sister.
Phoebe lives with her parents in New York. She is implied to be very close to his brothers, especially Holden, who holds her in high regard, thinking of her to be wise beyond her years much like their other sibling Allie who died of leukemia.
Phoebe symbolization of the childhood innocence Holden is trying to protect has been the common interpretation of her character.
In Salinger's earlier short story Last Day of the Last Furlough, the relationship between the characters Babe and Mattie mirror Phoebe's relationship with Holden.