Takeaki Kiryū, Tatsuo Nagumo's cousin, initially appears as an ordinary young man visiting relatives. His life shatters when the antagonist Faust Münchhausen II engineers a plane crash that kills his parents. Takeaki survives with life-threatening injuries, saved only by a blood transfusion from Tatsuo. This transfusion, carrying the essence of the Chōjin (Overfiend), preserves his life but initiates his corruption and supernatural connection, marking him for Münchhausen's schemes.

Münchhausen II identifies Takeaki as the potential vessel for Kyō-Ō, an ancient deity predating the Chōjin and capable of immense destruction. Through psychological manipulation, Münchhausen torments Takeaki with visions of his deceased parents, accelerating his mental collapse. This triggers sporadic physical mutations into a monstrous form driven by heightened aggression, leading to violent acts including rape and murder, orchestrated by Münchhausen exploiting Takeaki's grief to resurrect Kyō-Ō.

A significant counterpoint emerges in Takeaki's relationship with Megumi Amano, a man-beast hybrid. Genuine feelings develop between them, and Megumi attempts to shield him from Münchhausen. This bond introduces internal conflict, momentarily challenging Takeaki's descent. However, Münchhausen escalates his control, kidnapping Megumi and forcing Takeaki into violent confrontation with her. During a climactic battle on Shinjuku's skyscrapers, Takeaki resists Münchhausen's command to kill Tatsuo, demonstrating a fleeting reassertion of his humanity. This resistance is short-lived; to prevent further destruction, Megumi is compelled to kill Takeaki.

Following his death, Münchhausen transports Takeaki's body to the man-beast world. His corpse is impaled on the Gokumonkyō shrine, where his blood acts as a catalyst to awaken the Kyō-Ō idol, completing his role as a sacrificial vessel within the conflict.