Yūichi Niki is a student initially portrayed as nerdy and socially marginalized. He harbors an intense infatuation for his classmate Akemi Itō, which twists into a jealous and obsessive hatred for Tatsuo Nagumo after he becomes Akemi's boyfriend.

This volatile jealousy and craving for power attract the attention of supernatural forces. The demon Suikakujū identifies Niki's vulnerability and selects him as a vessel to destroy the Chōjin, the entity believed to reside within Nagumo. As part of this transformation, Niki is granted a demonic phallus, which he must use to replace his own through an act of self-mutilation.

This process reshapes his physique into a more muscular and powerful half-demon form, but it also corrupts his mind. He indulges in hedonistic and violent acts, taking revenge on those he once saw as tormentors. Occasional flashes of his original humanity surface, such as a moment of remorse after harming a sexual partner who reminds him of Akemi.

His pursuit of power and revenge leads to his death at the hands of Tatsuo Nagumo; in the anime, Nagumo tears out his organs, while in the manga, Nagumo's blood kills him.

In a later story arc, following the merging of the three worlds, Suikakujū's lover Fukakuki attempts to resurrect the demon within Niki's corpse using one of Suikakujū's horns. The resurrection is only partially successful due to the severe damage Nagumo inflicted on Niki's body, resulting in a weakened form of Suikakujū that must absorb other demons' power to regain strength.