Kirei - Body Like Iron

Confronted with such anger and despair, he was driven to
masochism. Under the pretense of penance for moral training, he
wounded himself repeatedly. But those tortures forged Kirei’s body
like iron. Before he realized it, he had risen to the top of the elite
of the Holy Church, as an Executor, a position few could follow
him to.

Fate/Zero, Prologue, Three Years Ago, Pg. 23