Yuichi Niki, a high school student marked by social alienation and simmering jealousy, evolves from a timid, mistreated youth into a figure consumed by darkness. His unrequited obsession with Akemi and bitter resentment toward Nagumo catalyze his fall, as opportunistic demons prey on his fragility, granting him inhuman power in exchange for his humanity—a pact sealed by the grotesque replacement of his genitalia with a demonic phallus. This visceral self-mutilation propels his metamorphosis into a half-demon hybrid.

In the manga, he appears as a junior high student entangled with gangs, directly receiving the demonic phallus from two shadowy entities before meeting his end via Nagumo’s lethal blood. The anime reimagines him as a college-aged pawn manipulated by Suikakujū to eradicate the Chōjin, culminating in Nagumo violently disemboweling him during their clash.

His tragic arc oscillates between fleeting self-awareness and unchecked brutality. A momentary realization of the pain he inflicts—mirroring his own past suffering—surfaces during a violent outburst, only to be crushed by his thirst for dominance. Visual cues, such as oppressive color shifts during scenes of abuse, underscore his fractured psyche.

Once meek, Yuichi’s descent into hedonistic cruelty starkly contrasts his earlier fragility, illustrating power’s corrosive grip. His physical evolution into a muscular, sadistic entity reflects his moral decay, while his eventual demise—a consequence of hubris—underscores the peril of ambition untethered from humanity.