A person who is indifferent to pains and pleasure of life:

The one word substitution is Stoic.

Sadist : the tendency to derive pleasure, especially sexual gratification, from inflicting pain, suffering, or humiliation on others.
Psychiatrist : a medical practitioner specializing in the diagnosis and treatment of mental illness.
Aristocrat : a member of the aristocracy.
Stoic : a person who can endure pain or hardship without showing their feelings or complaining.