Haruyoshi Kuga, once revered as his world's strongest onmyōji, faced betrayal by the Imperial Court and death at his own disciple's hands, victims of their fear towards his overwhelming power. With his final breath, he enacted the self-created "Curse of Reincarnation," seeking happiness in a new existence. This resulted in his rebirth as Seika Lamprogue within a European-medieval-style world.

Haunted by his past life's relentless pursuit of power and subsequent tactical blindness leading to downfall, Seika resolves to prioritize cunning over overt strength, avoiding attention while securing personal contentment. He retains memories and spiritual powers from his former life but lacks mana, rendering conventional magic of the new world unusable. Instead, he relies on onmyōjutsu, manipulating spiritual energy through talismans and elemental forces. His rare black hair and eyes contribute to a calm, unassuming appearance.

Seika exhibits calculated reserve, deliberately concealing his abilities to avert past tragedies. Initial distrust, fueled by his status as an illegitimate child shunned even by his noble family, gives way to fierce protective bonds with close companions like his servant Yifa and the prophesied Hero Amyu. His calm demeanor seldom breaks except in specific situations, such as enduring motion sickness or confronting threats to his friends.

His abilities center on onmyōdō, encompassing elemental control over fire, earth, water, metal, wood, yin, and yang – enabling spells like the phosphorus-based "Oni Flames Technique" and the toxin-infused "Poison Fire Technique." He summons shikigami and contracts ayakashi, utilizing paper talismans (hitogata) for barriers, curses, and healing; notable among these is the shapeshifting fox spirit, Yuki. Further skills include divination, curse manipulation (reflecting curses onto their casters), enhanced physical prowess, max-level exorcism, and sensory abilities like "Dragon Vein Vision," per in-universe assessments.

Key relationships define his path: Yifa, his childhood servant and first friend, represents unwavering loyalty; he views her as family, overlooking her romantic feelings. Amyu, the Hero destined to oppose the Demon King, becomes a trusted ally Seika mentors to become the world's strongest warrior, intending to live peacefully under her protection. This plan falters when his identity as the Demon King emerges, creating tension with her destined role. Yuki, his ayakashi familiar, provides companionship and strategic aid, recognizing his soul from their past connection.

His journey reveals a dual role as protector and antagonist figure. Early arcs show him enrolling at Lodonea Academy to support Amyu discreetly, yet political schemes force him into conflicts like foiling assassination plots against her. The revelation of his Demon King status – triggered when his soul occupies a vessel meant for the demon ruler – incites crisis, turning humans and demons against Amyu to prevent war. In the climax, he enacts mass resurrection after a rampage to free her, subsequently embarking on travels with Amyu, Yifa, and the redeemed mercenary Mabel.

His arc concludes with a shift from isolation to forging genuine bonds, wielding his powers defensively rather than for dominance, achieving contentment without repeating past mistakes.