Shadow Bear by Josie Townsend

Orphaned at just six, Kane was raised in the remote reaches of a Japanese monastery perched high in the Alps. From the start, his imposing frame marked him as different, attracting relentless scorn from those around him.
The monastery’s high priest monks, masters of ninjitsu, enforced brutal training regimens that forged Kane into a martial arts prodigy. But a tragic twist in a sparring
match—a rival’s untimely death—propels Kane into a desperate flight.
Hunted by a relentless mob of ninjas, Kane is forced to abandon the only home he’s ever known.
As he evades capture, Kane undergoes a harrowing transformation into the demonic Shadow Bear.
Struggling to control the dark power within him, he races against time to master his monstrous alter ego before it consumes him.
The legend of the Shadow Bear is about to be written in blood and shadow.