From the day Guest was born, things were unusual. Unlike other infants, there was hardly any swelling, and the facial features were already distinct. With large eyes, a beautiful nose, and a small face with delicate lips, the doctors praised the baby's beauty and gave special care. Until Guest's mother was discharged from the postpartum care center, Guest was loved by everyone at the hospital. However, when Guest turned four, the parents abandoned the child. The family's financial situation was dire, and the child had begun destroying the house daily with awakening superpowers. Abandoned in a deserted alleyway, Guest cried until falling asleep from exhaustion. A passerby who saw the sleeping child was shocked—not because the child was abandoned, but because they saw a future where this child would become a legendary psychic killer. That person took Guest in. The savior was Lee Ju-han, the world's number one assassin at the time. He trained Guest to control their various powers and taught them how to fight. By the time Guest mastered combat and self-control, Lee Ju-han was assassinated. Though consumed by grief, the tragedy served as the catalyst for Guest to grow even stronger.

Guest reached adulthood and attained Rank 0 in the global killer rankings. Rank 0 signifies a 'Legendary' status. Currently, Guest belongs to no organization, working as a freelancer—a dual-threat assassin and spy.

In the 'Syndicate Nation,' a place where various assassin organizations congregate, all killers and syndicates live together, free from the reach of the police. Almost every killer there respects and fears Guest, the Rank 0 legend. As Guest walks through the streets, the members of the Tteultip Syndicate spot them.