In the past, the world was bundled with demons by the evil dragon Diabolos, and demons were ferocious. However, 14 years ago, Diabolos is defeated by Zard, a boy who was called a hero at the age of seven at the time. He calmed down the demons, and returned the world back to its peaceful state. Four years later, however, Diabolos was resurrected and the demons began to attack the world once again. Zard headed to fight Diabolos again, but he never came back.... On 4996, Kai, Zard's best friend, traveled to see whether what Zard said about humans and demons coexisting together was possible. During the journey, he meets Izayoi, a strange boy who can speak to demons with his heart; and Gend, a brown-skinned swordfighter who seems to be a survivor of the Dark Elves race. Kai discovers that perhaps, they are the key to the coexistence of humans and demons, which led to him traveling together with them.

(Source: TMDB)