Doctor Gaines was a doctor working at Sioux Falls General Hospital.
This doctor was shown tending to two patients, one of them was Jody Mills. He complimented Jody on her recovery and left the two women to rest.
He met up with a little girl shortly after. The little girl wanted to become a doctor and grow up quickly, much to the doctor's confusion. The little girl proceeded to attack the doctor and morph into his form.
The little girl, who was in fact the vessel of a Leviathan, devoured the doctor and impersonated him in order to eat his patients too. For the remainder of the Leviathan's life, it kept Doctor Gaines' form.