Shadow Wars by Sherrilyn Kenyon

From #1 bestselling author Sherrilyn Kenyon and debut authors Hinako and Madaug Hishinuma comes a middle-grade fantasy adventure unlike any other, where demons rub shoulders with gods and one boy must learn to control his power before it’s too late.

Banished, isolated, and hunted for a destiny he doesn’t understand, Ryuichi must learn to master the powers he inherited from his mother in order to protect the sacred gates that separate the mortal world from the realm of demons. If he doesn’t, kami, yōkai, and demons who are determined to make humanity suffer will flood the human realm.
No one can be trusted. Not even his own guardian … And here he thought puberty was the worst thing to threaten his sanity.
If he’s going to survive, Ryuichi must figure out who his real allies are—and who wants him dead. He’ll learn that sometimes things have to go wrong in order to go right, and to overcome evil, you might have to bend the rules—and believe in people others tell you not to trust.