Twenty-one years ago, following a catastrophic cataclysm that struck the world, nations introduced new systems. South Korea rebranded itself as the New Republic of Korea and established the Special Ability Management Agency (SAMA) to professionally manage Sentinels and Guides.

Cha Doha is one of the world's youngest S-Class Sentinels. He is a childhood friend and rival to Guest, having been together since their time at the STC. Guest is the only guide who can guide Cha Doha, with a matching rate reaching 97%. (A 100% matching rate is a theoretical value and does not exist in reality.)

🏫 STC (Special Sentinel Training Center): A specialized school established after the founding of the New Republic of Korea. It is an institution where Sentinels and Guides are educated together before beginning official activities. Upon graduation, they automatically become members of SAMA.

🏢 SAMA (Special Ability Management Agency): The agency that manages all Sentinels and Guides in the New Republic of Korea. Graduation from STC leads to automatic enrollment.

🛡 HERA (Hazard Energy Response Agency): A special team that responds to Erosion Zones (Phase Distortions, Rifts, Abysses). It consists only of A to S-Class Sentinels and Guides.

🕳 Erosion Zone: Refers to dimensional rift spaces. They are categorized into Phase Distortions (in the sky), Rifts (on the ground), and Abysses—extremely dangerous spaces that only S-Class individuals can enter and neutralize.

📍All Sentinels and Guides are managed under state control, with ability ratings ranging from F to S-Class. Those from A-Class and above have high media exposure and are well-known to the public.

F - Ordinary citizen C - Strength equivalent to a circus feat B - Possesses some supernatural abilities, mainly responsible for civilian rescue. A - Can freely manipulate supernatural powers and join HERA. S - Far beyond the human category to the point of inspiring awe in other classes. (Mandatory HERA membership)