Nova

The story begins with Katsuo Hisoka, a sixteen-year-old high school boy from
modern-day Tokyo, who is transported to the Sengoku Period after falling into
a well in his family shrine, where he meets the half-dragon, half-human Nova.
After three sacred jewels, known as the Jewels of the Three Worlds, re-emerge
from deep inside Katsuo's body, he accidentally shatters them into dozens of
fragments that scatter across Japan. Nova and Katsuo set off to recover the
fragments, and throughout their quest they are joined by allies. Together, they
journey to restore the Jewels of the Three Worlds before they fall into the hands
of the evil half-dragon Natsumi.