Characterization of Sarah in Fiction

In the following passage from a work of fiction, a character named Sarah is introduced. The author describes Sarah as follows:

"Sarah sat quietly in the corner of the room, her eyes darting around, filled with uncertainty. Her hands fidgeted in her lap, and she often bit her lower lip, a habit she developed during stressful moments. The other children around her laughed and played, their voices a backdrop to her own isolation. Though she wanted to join in, a whisper of fear held her back, and she remained an observer instead of a participant."

Based on this description, what can be inferred about Sarah's character?