tsujinaka yoshiki.

Tsujinaka Yoshiki (辻つじ中なか佳よし紀き, Tsujinaka Yoshiki?) is a high school student in a rural village. Hikaru is his childhood friend. He is disturbed by the fact that Hikaru has been replaced by "Something".

Yoshiki is 175 centimeters tall (5'9"). He has short, straight black hair with long bangs that easily cover his eyes. After having his hair cut, his bangs fall above his eyebrows. He has three moles on his face: one underneath his right eye, one on his left cheek, and one underneath the left corner of his lip.
He is often depicted wearing his school uniform, which consists of black slacks and a white button down shirt, with dark gray-blue sneakers. Otherwise, he typically wears casual short sleeve shirts and shorts

Yoshiki is a quiet and reserved boy, though he becomes more talkative when discussing things he has a strong dislike for. His good friends describe him as secretive and having a tendency to hold back his thoughts even if they show plainly on his face. He isn't good at talking about himself.
According to the adults around him, his appearance and way he presents himself "makes the town a little darker". Although Yoshiki has friends he spends time with regularly, he feels disconnected from and suffocated by the people in his village to the point that he wants to leave or hide himself to avoid others prying into his life.
Yoshiki has great difficulty moving on from his best friend's death. This is primarily demonstrated by his decision to accept the creature that body-snatched Hikaru if it meant getting to resume his normal life with his friend. In addition to this, he often mournfully reflects on his memories with Hikaru and dreams about him. Finding Hikaru's body in the mountain has caused him to lose sleep. Because of his disability to let go, Yoshiki continues to choose to stay with "Hikaru" even when made aware of the danger it would pose to himself and the harm that "Hikaru" has done to other people. Yoshiki overall has a tendency to think negatively and become depressed, but because of his choice to stay with "Hikaru" this quality of his is compounded.

Yoshiki is able to see and hear impurities. However, his innate spiritual sensitivity and insight is not very strong. Had he not discovered Hikaru's body in the mountains, he would not have realized that he'd been replaced by "Hikaru". He would've just attributed any differences between Hikaru and "Hikaru" to the head injury from his fall in the mountains.

Illegitimates (落おとし子ご, Otoshigo?, lit. "illegitimate child") are a type of entity from The Other Side. Little is known of their true nature, but Illegitimates are immortal entities that originate from The Other Side and cross into This World without warning (although their rate of appearance is incredibly low). The first recorded appearance occurred around the 14th century. Illegitimates are undying. Based on "Hikaru", even if they are gravely injured, Illegitimates are capable of repairing themselves to their normal state. They completely lack a sense of pain unless they are significantly weakened somehow.

Impurities (ケガレ, Kegare?) are entities similar to ghosts or spirits of people who have died. They collect in places where humans live, although they typically do not show themselves to most humans. Impurities are an accumulation of the souls of the dead and the strong emotions of the living, which have mixed together within The Other Side and were then reborn back into our world. It is noted by Kurebayashi that the souls of the dead are still preserved enough to recognize them within the impurity.

The Other Side (向むこう, Mukō?, lit. "opposite side; beyond") is the world of the non-living. The Other Side is not a physical location, but rather a non-physical realm that overlaps This World. The two realms appear mostly identical to one another. This World and That World are essentially laid on top of one another, only separated by a thin "curtain". Strong emotions of the living may mix together with the souls on The Other Side, allowing them to be reborn as impurities.

Holes (穴あな, Ana?) are passageways between the world of the living and The Other Side. Holes are created when distortions spontaneously form in the "curtain" that separates the world of the living and The Other Side, allowing entities such as humans, impurities, or illegitimates to pass through.