Keiji Mogami, originally celebrated as "This Century's Greatest Psychic," gained fame demonstrating his powers on Japanese television roughly 40 years before the main story. He initially dedicated himself to benevolent acts, exorcising evil spirits and offering guidance. However, the financial burden of his mother's grave and severe illness forced him into accepting morally questionable underworld assignments, including assassination contracts, to fund her care. Despite his efforts, his mother died and returned as an evil spirit, revealing that the negative energy and "demons" drawn by his actions had caused her suffering. This revelation shattered Mogami's worldview, convincing him of humanity's inherent corruption. Abandoning altruism, he focused solely on absorbing spiritual energy to become an immensely powerful evil spirit after death. He traveled widely, consuming evil spirits and nearly destroying entities like the malevolent spirit Dimple and his followers. Approximately 25 years later, Mogami committed suicide in a broadcast event famously known as a live accident, solidifying his transformation into a vengeful spirit. As an evil spirit, Mogami sought to punish those he deemed corrupt or exploitative, especially wealthy and powerful individuals escaping consequences. His methods grew increasingly extreme, involving psychological torture and physical possession. He notably possessed Minori Asagiri, daughter of a wealthy businessman, aiming to lure and eliminate psychics he considered fraudulent. He tormented Minori and her father psychologically, even orchestrating the father's stabbing to maintain control. This scheme attracted numerous psychics, including Shigeo Kageyama and his mentor Arataka Reigen. Mogami developed a specific fascination with Shigeo due to his immense power and perceived naivety. Trapping Shigeo within a complex illusory world inside Minori's mind, he subjected him to six months of relentless bullying and powerlessness, attempting to corrupt him by proving the world's inherent cruelty. Although Shigeo initially succumbed to rage, he ultimately rejected Mogami's nihilism, embracing gratitude for his real-life relationships. Mogami accepted this defeat calmly, expressing interest in observing Shigeo's future path. Later, after capture and sealing by the psychic Matsuo, Mogami resurfaced during Claw's world domination attempt. Briefly confronting the esper Minegishi, he argued irredeemably corrupt individuals deserved eradication. Shigeo intervened, advocating faith in humanity's capacity for change. Mogami relented, releasing Minegishi while cautioning Shigeo about the necessity of harshness for true strength. His subsequent fate remains ambiguous, with Dimple speculating he may have moved on to the afterlife. Mogami possessed a vast range of psychic and spiritual powers, including telekinesis, energy absorption, biological manipulation, possession, and soul manipulation. He could create intricate, reality-warping mental landscapes, like the six-month illusion designed to break Shigeo. As a spirit, he demonstrated high durability, surviving attacks capable of destroying city blocks, and could merge with or control thousands of other spirits to amplify his power. His personality evolved from a jovial, supportive figure to a bitter, calculating entity driven by a warped sense of justice. Despite ruthlessness, he retained capacity for introspection and empathy, particularly towards Shigeo, whom he saw as a reflection of his former self. This complexity manifested in his acceptance of defeat and final departure, suggesting lingering ambiguity in his worldview.