Naraku Ninja is a mysterious and ancient ninja soul that serves as the source of power for the titular protagonist of the story, possessing the salaryman Kenji Fujikido after his wife and child are killed in a ninja turf war. As a noncorporeal entity with its own will, Naraku Ninja is defined by an intense craving for massacring ninja, a drive that aligns with Fujikido's personal lust for revenge against the Soukai Syndicate. This convergence of desires brings the persona known as Ninja Slayer into existence, a terrifying ninja-killing grim reaper. The relationship between Naraku Ninja and Fujikido is one of symbiotic possession: the more their mental resonance and physical symbiosis deepens, the more powerful Ninja Slayer's karate becomes. However, there is a critical risk – if Fujikido completely surrenders his will to Naraku Ninja, his karate grows nearly invincible, but Naraku Ninja would then be able to massacre freely without restraint. Naraku Ninja's abilities include granting its host enhanced ninja-level physical attributes, formidable karate techniques, and the ability to generate black flames. This dark power constantly pressures the host to allow its full takeover, and if a host ever completes his goal of revenge and loses his purpose, he will be consumed by Naraku's flames and die, after which the soul departs to find a new host. Naraku Ninja thus acts as both the enabler of Fujikido's vengeance and a constant threat to his identity, embodying a primal, destructive force that demands ever-greater violence.