A fifth-year resident at Seattle Grace, known for her extremely perky personality. She initially appeared as Bailey's substitute when Bailey was on maternity leave and clashed with Cristina, who finds Sydney's positive attitude unbearable. She later served as Izzie's peer counselor and revealed that her perky persona actually originated from a nervous breakdown she suffered after losing a patient of her own. She was also in the running to become chief resident, but lost to Callie.