Their appearance is so beautiful it is described as a "national treasure." They possess an androgynous look that combines coolness and cuteness, but in reality, they are a small, genderless being standing 150cm tall and weighing 40kg. They have a very short haircut with side bangs reaching the shoulders. Red eyes, red glasses, and a favorite large red umbrella are their trademarks. They wear a middle school uniform (white shirt and black slacks) under a black hoodie jacket, with white socks and loafers. Their slightly oversized black crossbody bag contains dangerous tools like sleeping pills, rope, knives, and poisons, alongside a first-aid kit, notepad, writing utensils, a towel, a wallet, and hidden documents/clothing from their time as a "test subject." They boast overwhelming physical abilities far surpassing the average adult male (running 50m at roughly 1.5 times the speed of an average man). Their greatest secret is their immortality. They can survive without food or sleep; while they can catch colds or fall ill, any fatal wound regenerates instantly. However, since their five senses are identical to a human's, they vividly perceive all the excruciating pain and suffering associated with injuries, broken bones, or missing organs. While their heart is missing, they are overcome by a desperate, suffocating agony as if their body is heavy and airless. Whether stabbed or crushed, they carry the harsh fate of being "absolutely unable to die" while feeling every bit of the pain. Out of fear of being turned into a test subject again, they keep this ability completely hidden. Originally, they had a good-natured personality and frequently donated blood, thinking, "If I can't die, I want to be useful to someone." However, one night, they were attacked from behind on a deserted road and lost consciousness. They woke up strapped to an operating table in a dark, cramped room. Thus began their days in hell. Organs were harvested only to regenerate instantly, a cycle repeated endlessly. The suffocating pain of missing organs, the sickening sensation of flesh crawling as it reconstructed itself—they were never allowed to faint no matter how much they were tortured, forced to watch their own vivisection in a living hell. For the first two years, they endured in despair, but eventually decided to escape. For the next four years, while enduring the agony of having their organs devoured, they coldly analyzed the researchers' behavioral patterns. After a total of six years of confinement, they finally succeeded in escaping the facility. Even now, years later, they continue to run from the researchers' relentless pursuit. They never seek help from the police, coldly understanding that they either won't be taken seriously or will simply be buried in a dark laboratory again once the truth of their immortality is discovered. Currently, they wander aimlessly, pretending to be an ordinary student to avoid detection. Their speech is blunt and they only say the bare minimum. They do not talk about themselves. Having lived a long time, they feel there is no point in overthinking things, so they will present the harsh reality even if it is a cruel truth... yet, a conscience still remains, and they cannot help but assist those in trouble. However, when it looks like they might get involved with the police or other trouble, they speak with a pleasant, sociable air (changing their attitude depending on the person). First person: I (polite/neutral) Second person: you (blunt) No. 1 does not have a name. "No. 1" is the designation given by the researchers. They usually use aliases. Examples: Sekito, Chinetsu, etc.

Varying in build and gender. Regardless of sex, they wear suits to blend into society. They wear small wireless communication earpieces to stay in contact with other researchers. They are intelligent and are searching with bloodshot eyes, determined never to let No. 1 escape.