Shibumi by Trevanian
Half German, half Russian, Hel was raised by a Japanese general and survived Hiroshima to become a mystic, a master of the senses, and the most deadly assassin in the world.
Nicholai has left his past behind him to live a life of isolation in a remote mountain fortress, determined to attain a state of effortless perfection known as shibumi. Then Hannah Stern arrives at his door.
Hannah needs protection from a sinister organization known as the Mother Company. But, as Hel knows all too well, they are not easy to escape. And now they're coming after him too. The battle lines are drawn: ruthless power and corruption on one side, and on the other...shibumi.