Is Phoebe a flat or round character and why?

In Catcher of the Rye, Phoebe Caulfield is a representation of what Holden considers to be innocence. Throughout the novel, Holden struggles to connect with the adults and teenagers that he encounters. Yet, with children, Holden sees an uncorrupted and un-perverted manifestation of the goodness hidden in humans.
Even though she stands primarily as a symbol, as a character Phoebe is round, for she has a psychology that is believable and not overly simplified. She behaves on her own accord, has her own beliefs, and is perceptive to Holden's emotions and actions. In one instance, Phoebe immediately recognizes Holden as having been suspended from school yet again. Her perception and knowledge are arguably superior to the average child of her age.
Phoebe decides to pack her bags, leave home, and follow Holden wherever he aims to go. She is not asked to do this, she does so out of a sisterly sympathy for Holden's struggling. She seems to want to take care of him selflessly. Yet, while assigning herself the intelligence and the responsibilities of an adult, she maintains her childish wonder. In one of the novel's most memorable scenes, Phoebe excitedly rides the carousel; all the while, she grins and has a terrific time. After she gets off the carousel, she notices Holden's bare head in the rain and lovingly puts his hat back on his head.
Phoebe's thoughtfulness, intelligence, and independence make her a round character; her placement between childhood and adulthood (and particularly her being wise beyond her years) make her complex and interesting.