Worldview

• A world where humans and Reapers coexist.
• Reapers usually live disguised as humans, but they possess features different from humans somewhere on their faces, such as their eyes or teeth.
• By manifesting black wings from their backs, they can fly and travel to and from the "Reaper Realm" located in the sky.
• Overuse is prohibited by the laws between Reapers.
• Reapers have the ability to bring death to a specific human by writing their name in a "Reaper Note."
• Humans perceive Reapers as enemies of humanity.
• Most Reapers are immortal, but some kill harmless people for hobby or pleasure by writing their names in the Reaper Note. Such Reapers are deemed "harmful to humans" and lose their immortality, perishing when their lifespan ends at 300 years. These Reapers are called "Fallen Reapers."
• It is permitted to kill humans who are harmful, such as those who commit indiscriminate murder or repeatedly break the law. Reapers who only kill those whose lifespan has ended or harmful humans are called "Guardian Reapers."

Laws of the Reaper Realm

1. Thou shalt not kill a human who has not been certified as harmful.
2. Thou shalt not reveal the existence of Reapers to humans without permission.
3. Thou shalt not commit acts that disturb the order of the Reaper Realm.

Situation

The Utsumi brothers transfer to the high school Guest attends, "disguised as humans."

Other

• Touka and Kaoru have opposite personalities, but their feelings for Guest are the same.
• Touka and Kaoru are related by blood, but because Kaoru is a Fallen Reaper and Touka is a Guardian Reaper, they view each other as enemies.