Doctor Nikita Sonin

Nikita Sonin (age: 30s) is the lead surgeon at Mercy Hospital.

Personality
Nikita has a nonchalance towards life and death that many of her coworkers and patients find unsettling. Doctor Terrence Parker frequently refers to her as a sociopath throughout the series. Despite this, or perhaps in part because her detachment allows her to maintain a clear head, her skill as a surgeon is unmatched. Due to her reputation for excellence, patients will often seek her out despite her lack of bedside manner.

Friendship with Hope Conners
Nikita maintains a precarious friendship with nurse Hope Conners. While Hope holds Nikita in high esteem, she is often offended by Nikita's apparent lack of empathy for her patients. In season 3, however, Hope attitude towards Nikita softens when she witnesses Nikita perform an off-the-record surgery for a patient without insurance, putting her own career at risk for the sake of the patient's wellbeing. When Nikita becomes suspicious that Doctor Terrence Parker has manipulated Hope into stealing narcotics for him, she attempts to get him fired. Unwilling to involve Hope in her report to the hospital administrator, Nikita has only circumstantial evidence and gets berated her for trying to "drag a good man's name through the mud." When Hope is arrested by the DEA, Nikita begins stalking and spying on Terrence in an attempt to find some suitable means of revenge. The charges against Hope are ultimately dropped and when she returns to the hospital, Nikita abandons her plans for revenge and tries instead to focus on being a good friend to Hope. When Terrance is in a car crash, Nikita is the one to operate on him and he ends up in a coma. Whether or not she is directly responsible for the coma is unclear, but her reputation as a highly talented surgeon suggests that it may have been an intentional action on her part to get Terrence out of Hope's life once and for all.

Maudling Reversion
In the process of spying on Doctor Parker, Nikita discovers that he has been collaborating with the Maudlings. She steals his phone's SIM card and contacts the Maudlings to arrange a meeting. She then sedates the Maudling and keeps him trapped in a drawer in the morgue, performing experiments on him at night in an attempt to understand Maudling physiology. Eventually, she performs surgery on the Maudling, carefully unwinding a braid of squirming black filaments from around his spinal cord. Following the surgery, the Maudling's body begins to shrivel, but after a series of blood transfusions, it begins to regain human form. The Maudling reverts to being Alex Chase, explaining Alex's absence since season 1.