Who is Phoebe Henry from The Memory Keeper's Daughter and what is their importance?

As a Downs syndrome child, little Phoebe goes through life in her own small world, unaware for the most part that people notice her differences and that she makes people feel awkward. She is innocent and loving, and understands most of the basic concepts that Caroline and Al have taught her. Phoebe loves cats and weaving; Caroline has made sure that Phoebe has a full life in spite of her slowness. But Phoebe is sadly naive, and a potential target for people with bad intentions. Caroline allows her to ride the bus alone for a while, but when a man tries to talk Phoebe into going home with him, Caroline no longer lets her ride the bus. They establish a secret word to distinguish strangers from friends.