Koichi Tsujii is a supporting character in the anime Junji Ito Maniac: Japanese Tales of the Macabre. He is the older brother of Souichi Tsujii and a recurring figure in the segments that center on his younger sibling. As the eldest son in the Tsujii family, Koichi is defined by his level-headed, responsible, and highly patient nature. He is the complete opposite of his brother in temperament, described as a diligent and serious young man who possesses a strong sense of duty and compassion. This caring side is evident in his willingness to help strangers in need, contrasting sharply with Souichi's gloomy and malicious disposition.

Koichi's primary motivation is to pursue a normal, quiet life, and his most immediate goal within the story is to focus on his academic studies. This desire for peace and personal space directly drives the conflict he faces, as it makes him the primary target of his brother's disruptive schemes. His role in the narrative is that of the grounded straight man and the principal victim of Souichi's supernatural pranks. He represents the perspective of an ordinary person caught in bizarre domestic horrors that are largely engineered by his eccentric sibling.

His most significant relationship is with Souichi. Their dynamic is a contentious sibling rivalry, with Souichi constantly subjecting Koichi to petty curses, harassments, and manipulations. Koichi also shares a bond with his sister Sayuri, as the two of them often find themselves as a united front enduring Souichi's chaotic antics together. Despite being a frequent target, Koichi does not experience major character growth or develop any supernatural abilities. Instead, his function is to remain a consistent anchor of normalcy, patience, and decency. By staying steady while the world around him grows increasingly strange and threatening due to his brother's actions, he underscores the absurdity and persistence of the horror he must endure.