The Sword of Kuromori

The Sword of Kuromori is the first book in the Kuromori trilogy.

Summary
Kenny Blackwood arrives in Tokyo to spend the summer with his father and is stunned to discover a destiny he had never dreamed of when he finds himself in the middle of a hidden war that is about to explode. Racing against an impossible deadline, Kenny must find the fabled Sword of Heaven and use it to prevent the disaster. But a host of terrifying monsters is out to destroy him, and success will come at a price. With clever, fearless, sarcastic Kiyomi at his side, Kenny must negotiate the worlds of modern and mythic Japan to find the lost sword, before it's too late.