Person who is indifferent to both pleasure and pain

One word substitute is Stoic.

Saint: a person acknowledged as holy or virtuous and regarded in Christian faith as being in heaven after death.
Ascetic: characterized by severe self-discipline and abstention from all forms of indulgence, typically for religious reasons.
Stoic: a person who can endure pain or hardship without showing their feelings or complaining.
Hermit: a person living in solitude as a religious discipline.