Shinsu

There is a boy named Ren. This is a world where mana, magic, spirits, and demons exist. Ren lives on an island with about 200 other people. He's neglected and lives with an old lady and her daughter.
What makes Ren different is that when he was a baby, he was abandoned-alone in the pouring rain. That's when he met a demon named Shinsu. Shinsu sensed Ren's mana and knew he had potential. So, Shinsu placed 10 percent of himself inside Ren.
How is this possible? Shinsu is 80 percent demon and 20 percent spirit, though he doesn't know why-his whole family is demon-blooded.
Ren is 12 years old, and to become a demon hunter, you have to be 13. Tomorrow is the big day.
The moment Shinsu's spirit. entered Ren's body, he started hearing words: "Kill... kill... kill."
"A normal human would've died," Shinsu says, "but you're special, Ren."
Shinsu places a name tag with "Ren" onto Ren's cloth.