She is a shy, independent, considerate child. She enjoys being in the country with her grandmother, as opposed to the busy city. She is content with the simple things in life, like the beautiful countryside and her only companion, the cow Mistress Moolly.
She is also very mature, and makes a hard decision in the end to preserve the bird, at the sacrifice of her only human friend in quite some time. She values the beauty and life of the creatures around her over the pleasures of human companionship.
Her maturity and independence stand out. For a girl her age, those two traits are pretty rare. Most often, teenage or preteen girls value sociality, friendship, and peer-approval, and put all else above having those things. But Sylvia is unique; she relishes being alone, is afraid of most human interaction, and makes a tough decision at the end that sacrfices a tender friendship.