The Kaisho  (English, Electronic book text, Lustbader Eric Van)

SHADOW WARRIOR. The ancient Japanese art of death is practiced by a select few. Of those young men who are chosen to train in the way of the shadow warrior, most will fail. Those who pass these trials, the Ninja, become the silent assassins, lonely guardians, and unseen watchers of legend. But rarer still are those masters of the shadow craft, the Shiro - those whose fearsome abilities are matched only by their rigid code of honour. Men like Nicholas Linnear... THE KAISHO. The samurai were the nobility of the warrior class. But if a samurai fell into disgrace, he became a ronin - a mercenary without state or master. From these outlaws came the bandit clans, the Yakuza. Today the world's largest criminal syndicate. Now Nicholas Linnear, a shadow warrior bound to tradition and honour, finds himself forced by an old oath to protect the head of this vast network. The man they call the Kaisho.