Kamen Rider Hibiki (2005)

Teenager Asumu Adachi is at a crossroads. He's about to start high school and unsure of what he wants to do with his life. And as if puberty & homework weren't bad enough, he stumbles upon a huge secret: his new friend Hibiki is secretly an oni! Hibiki & his oni comrades are the only thing stopping the monstrous Makamou from devouring mankind.

As Asumu is pulled into a secret war, Hibiki & his allies teach the teenager what it means to be an adult in a uncertain world.