Lin Koujo hails from Hong Kong and serves as both assistant and guardian to Kazuya Shibuya at Shibuya Psychic Research. Hired by Naru's parents, his duties involve teaching Naru to control psychic energy through qigong and protecting him until adulthood. He also aids Naru's search for his deceased twin brother, Eugene, sharing a childhood connection with the twin.

Standing 203 cm tall, Lin possesses long black bangs that conceal his right blue eye, leaving only his left grey eye visible. He favors formal attire. This hairstyle manages his "electromagnetic vision," granting perception of light beyond normal human capacity; using both eyes simultaneously causes discomfort.

Reserved, solemn, and taciturn, Lin rarely speaks unnecessarily. He initially displays hostility towards Japanese people, rooted in historical conflicts, particularly evident in interactions with Mai Taniyama. Their relationship shifts after a confrontation in volume 6, where Mai challenges his prejudice, urging judgment of her as an individual rather than by nationality or orphan status. Lin laughs, noting Eugene once said the same, leading to mutual respect.

A practitioner of Chinese Taoist sorcery and Onmyōdō, Lin demonstrates expertise in exorcism, spirit summoning, and controlling captured spirits as familiars (shikigami). These abilities make him one of the team's most formidable members during paranormal investigations.

His character develops through relationships. Beyond protecting Naru, he shares a friendly rapport with Madoka Mori. Initial friction with Naru over ethnic differences gradually transformed into a bond of trust and reliance, making Lin Naru's primary source of friendship and operational support.

The sequel *Akumu no Sumu Ie* confirms his full name as Lin Koujo (林興徐) and reinforces his SPR investigator role. His reserved nature results in a faint presence during investigations, consistent with earlier descriptions of him as "shadow-like" or a "demon when stepped on," referencing the rules of Shadow Tag.