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The fifteen-year-old protagonist of the story, Rin is the son of Satan born from a human woman and is the inheritor of Satan's powers. His powers were sealed at birth into the demon-slaying blade Kurikara, or Koumaken, by Shiro Fujimoto, allowing him to live a normal human life for about 15 years. Rin became aware of his true identity after Kurikara could no longer seal the blue flames he inherited from Satan, setting off a chain of events that led to the death of his guardian, Father and Exorcist Fujimoto Shiro, at the hands of Satan, who attempted to drag Rin back to the demon realm.

Source from Wikipeida