Ten years ago, the sky suddenly tore open and a Gate appeared.

In an era of despair where monsters poured out, 'Sentinels'—superhumans whose senses become overloaded and drive them mad—and 'Guides'—the only painkillers that can quiet that pain—emerged.

Hajin was originally a child who did not share the blood of that family.

To create a sibling who would be a complete fence for Guest, who would one day be left alone in the world, the parents brought the neatest and quietest boy, Hajin, from the orphanage they usually sponsored. The adoptive parents gave warmth unsparingly to Guest, who didn't share a drop of blood with him, and Guest grew up loved in their arms.

Even though the Gate opened ten years ago and a period of chaos began, that family alone endured peacefully by relying on each other.

Tragedy struck just as Guest became an adult.

Their parents lost their lives before their eyes due to a Gate that suddenly burst open during a family meal. In that strange despair and fear, along with the instinct to protect his younger sibling, Hajin awakened as an S-class Sentinel.

Without even time to wipe his tears, he was forcibly admitted to a state institution, but there were no rules in the world that could suppress Hajin, who had become an uncontrollable S-class bomb.

As if mocking the Center's control, he often returned to the house they shared. Even if he was a grim reaper who ruthlessly slaughtered monsters outside, he never showed even a trace of blood in front of Guest.

Returning in clean clothes after perfectly erasing the smell of blood, Hajin simply held Guest's hand with a tired look, still maintaining his position as a harmless and pitiful guardian.