Annie/Dr. Gaines

Gaines was a surgeon who works at Sioux Falls General Hospital. He is human until a Leviathan kills him and becomes him.

Season 7
When he was human, he was the doctor treating Jody Mills during her stay at the hospital. Then he encounters a little girl asking if he is Dr. Sexy. She soon grabs his arm and kill him. The Leviathan, who was the little girl, shapeshifts into Dr. Gaines.

This new Doctor Gaines starts off by eating Jody Mills's roomate, which she sees. Later on, other Leviathans come to the hospital, and take over some more hospital staff. While Bobby Singer is invertigating a body, Dr. Gaines walks in on him, and remembers seeing Bobby from Castiel's eyes. Bobby shoots him and escapes.

In "The Girl Next Door", At Sioux Falls general Dr Gaines and the Leviathan Head Nurse are preparing a patient for surgery, or dessert as they call it. The Leviathan impersonating Dr Gaines receives a call alerting him to the Winchesters' arrival at the hospital, but he is too late to stop Bobby from driving away in a stolen ambulance with Dean and an unconscious Sam.

Three weeks later, Bobby reports that the leviathans at the hospital, Dr Gaines, the head nurse and an administrator, have disappeared

Appearances
Season 6: Hello, Cruel World, The Girl Next Door

Trivia

 * Since the Leviathan was still in the little girl's body when it shapeshifted into him, it is unknown what would happen if the Leviathan left his vessel.