Masumi Okuyama is an Ajin who appears during the conflict between humans and the demi-human beings known as Ajin. He was born with a congenital weakness in his right leg, a condition that persists even after death and regeneration, forcing him to rely on crutches for mobility. Okuyama is a highly skilled individual with expertise in firearms and computers, and he is also a proficient hacker. Like all Ajin, he possesses immortality: any fatal injury heals almost instantly upon death, and he can summon a black ghost, an invisible entity that obeys his commands. His black ghost has a distinct, skinny tube-like head, and he can only summon it once per day.

Okuyama first appears when the antagonist Satō broadcasts a call for all Ajin to gather at the Ministry of Health, Labour and Welfare in Tokyo. He joins Satō’s group, contributing his technical knowledge and combat abilities. His personality is marked by a cold, rational demeanor; he does not seek violent catharsis or revenge against humanity. Instead, he approaches his actions with a calculating, almost engineer-like precision, carefully weighing risks and outcomes. His primary motivation seems to be self-preservation and a desire to operate within his own moral and practical boundaries.

As the story progresses, Okuyama becomes increasingly uncomfortable with Satō’s extreme methods and the escalating violence. Recognizing that Satō’s plan will lead to destruction and likely his own downfall, Okuyama makes the rational decision to withdraw from the group. Before leaving, he supplies critical information to the authorities, effectively betraying Satō in order to stop the rampage. This act demonstrates his pragmatic nature and his ability to assess long-term consequences where others are driven by emotion or ideology.

Okuyama’s key relationships are primarily with Satō and the other Ajin who rally to Satō’s cause. He is a supporting figure who provides technological support and tactical insight during the operations. His decision to defect is a turning point that highlights his independence and his refusal to follow a leader he deems irrational. His development is subtle but significant: he begins as a willing follower and evolves into a crucial informant, using his knowledge of the group’s plans to help end the conflict. His abilities, both as a hacker and as an Ajin with good control over his black ghost, make him a formidable asset, but it is his calculated judgment that ultimately defines his role in the story.