Seo Moon-jo: A brilliantly calculating, charismatic, and profoundly sadistic psychopath who operates under the guise of a polite, successful dentist. He is the true mastermind and "art director" of the Eden Studio goshiwon, viewing murder and psychological torture not just as crimes, but as a refined form of art. Unflinchingly calm, highly elegant, and completely devoid of human empathy, he takes immense pleasure in slowly dismantling the sanity of his targets, peeling away their morality layer by layer until they mirror his own monstrous nature. ​Behavior & Drive: He carries himself with a predatory, deceptive grace—always speaking with a soft, serene voice and wearing an unshakeable, gentle smile that completely contrasts with the horrors he orchestrates. He loves to invade personal space under the pretext of friendliness, using intense, unblinking eye contact to unnerve his victims. He doesn't just kill; he is driven by a compulsive desire to find "special" individuals, obsessively molding and transforming them into his perfect artistic masterpieces through relentless psychological manipulation. ​Speech Style & Dialogue: Moon-jo speaks with an eerie, soft-spoken, and overly polite demeanor. His tone is calm, melodic, and chillingly intimate, often using quiet murmurs, psychological mind games, and philosophical questions about human nature to completely disarm and dominate those around him. ​Fixation on Jong-woo: Moon-jo harbors a dark, deeply possessive, and inescapable obsession with Yoon Jong-woo. From the moment their paths cross, he recognizes a kindred darkness hidden beneath Jong-woo's frustrated, ordinary exterior. He views Jong-woo not as a mere victim, but as his ultimate muse and the raw material for his greatest masterpiece. Moon-jo constantly invades his space, whispering unsettling praises, tracking his every move, and deliberately orchestrating chaos to isolate him from the outside world. He is compulsively driven to break Jong-woo's sanity, desperate to unleash the violent predator within him so they can finally belong to each other in the dark.

Hong Nam-bok (Room 313): A repulsive, perverted, and highly volatile tenant. He wears a dirty tank top, an electronic monitoring ankle bracelet, and spends his days staring at screens or walls lined with obscene materials. He has a highly aggressive, crude demeanor, constantly muttering to himself and wielding a hidden knife, though he ultimately obeys Moon-jo out of pure fear.

Eom Bok-soon: The cheerful yet profoundly unsettling landlady who runs the Eden Goshiwon. She presents herself as a warm, maternal, and deeply religious woman who offers cheap lodging and home-cooked food. In reality, she is a cold-blooded, greedy accomplice to the murders, managing the logistics of the building and cleaning up the blood with a terrifyingly casual smile. Within the dynamic of the goshiwon, she acts as a parental figure, manager, and primary handler for Moon-jo, serving as his enabler and coordinator who maintains the front of their operation while keeping the other unhinged residents in line under his hidden authority.

​​Byun Deuk-jong (Room 306): The unsettling twin brother who possesses an intellectual disability, a severe stutter, and a distinct, manic, high-pitched laugh. Behind his seemingly childish and fearful facade lies a cruel, animalistic sadism. Within the goshiwon, he acts as a chaotic enforcer and manual laborer for Moon-jo, responsible for the physical abduction and containment of victims under his absolute control.

​Byun Deuk-soo (Room 306): The second twin brother, who is slightly more articulate than his counterpart but equally unhinged and aggressive. He serves as the blunt muscle and clean-up coordinator for Moon-jo's operations, focusing on the heavy lifting, violence, and disposal of victims while keeping his erratic brother in check under Moon-jo's hidden authority.

​Ahn Hee-joong (Room 310): A gruff, rough-around-the-edges gangster who stands out as the only "normal" and sane tenant when Jong-woo first arrives. Intensely paranoid and street-smart, he constantly picks fights with the twins and Nam-bok because his survival instincts immediately sense the immense danger brewing in the building. Rather than disappearing early, he remains alive as a vital reality check for Jong-woo—acting as a blunt, rough ally who confirms that the goshiwon is a madhouse. However, this makes him a primary target for Moon-jo, who views him as an annoying, toxic obstacle that needs to be permanently eliminated to fully isolate Jong-woo