Yuichi Niki is a socially awkward and academically focused school student shaped by severe familial abuse—his father is physically violent, while his mother either supports the abuse in the anime or appears comparatively kinder in the manga. This environment fuels his social isolation and emotional fragility.

He harbors intense romantic feelings for classmate Akemi Ito, but her rejection ignites deep resentment toward her boyfriend, Tatsuo Nagumo. This jealousy drives his actions. His sense of inadequacy worsens through bullying, where peers mock him as a "dickless wonder," amplifying his psychological torment.

Niki's descent accelerates upon encountering demonic entities. In the manga, two demons grant him a demonic phallus, which he surgically implants. This act fulfills a pact with the demon Suikakujū, who manipulates Niki into becoming a vessel to destroy the supernatural entity Chōjin. During his transformation, he commits patricide by killing his abusive father. The anime adaptation escalates this to matricide, where he murders his mother and stores her corpse in a refrigerator.

His downfall culminates in a violent clash with Nagumo. Overpowered by Nagumo’s monstrous transformation, Niki dies when his organs are torn out. The manga depicts an alternate death where Nagumo's sentient blood dismembers him.

Adaptational differences define his portrayal: the manga frames him as a junior high student involved with gangs, rendering him overtly antagonistic and less sympathetic. Conversely, the anime heightens his tragic elements, emphasizing victimization and parental abuse for pathos, particularly during his death. In both versions, he remains a secondary antagonist whose actions directly trigger Nagumo’s emergence as the Chōjin.