Demon King Rasen is an extraterrestrial Yo-kai entity and leader of the Onimaro, originating beyond Earth. He arrives via comet every thousand years, heralding an invasion where his followers exploit human malevolent emotions and negative impulses. This infection transforms humans into mutated Kaodeka Oni, spreading uncontrollable chaos. Rasen's primary motivation is consuming human hearts to amplify animosity, causing death or violence, aiming to dominate both human and Yo-kai worlds. Rasen targets individuals harboring deep-seated resentment or isolation, like the junior high schooler Touma Tsukinami. Three lesser Onimaro approach Touma, offering power through possession. After Touma accepts, they summon Rasen, who forcibly merges with him. This grants Touma superhuman strength and the Kigan Gear, a wrist device enabling control over other infected humans. Rasen manipulates Touma's desire for recognition, promising world acknowledgment while exploiting his pain. As Touma deteriorates, Rasen eventually abandons him, intending to kill him once deemed unfit. Physically, Rasen manifests as a colossal armored figure with pinkish skin and purple-green armor adorned with three distinct Oni faces on its shoulder pads, interconnected by black chains. He wields immense destructive power, including city-leveling energy projections. His near-invulnerability requires the combined forces of protagonists Natsume Amano and redeemed Touma, Yo-kai allies including Kitaro and his cohort, and the fused power of Yo-kai World rulers Enma and Ananta to overcome. Their assault shatters Rasen's armor, culminating in his decapitation via the Fudou Raimeiken sword. Even beheaded, Rasen attempts a final attack, impaling Touma before full destruction. His defeat necessitates the sacrifice of the three Onimaro followers, who revive Touma using their own lifeforce. A variant, Oniou Rasen Kegare, appears as a sentient clone corrupted by darkness. He retains Rasen's belief in absolute power but displays a more reflective demeanor, acknowledging the heroes' strength and accepting defeat gracefully. Unlike the original, he harbors no ill will and permits the protagonists to destroy Puni-gami Kegare—the being controlling him—knowing it causes his own death. He rationalizes this by affirming the "true" Rasen's existence transcends his own.