Seika Lamprogue began as Haruyoshi Kuga, the mightiest onmyouji of his original world, betrayed and slain by his own disciple in a political conflict. He activated his self-devised "Curse of Reincarnation" to be reborn elsewhere, seeking a quiet, unnoticed life. Reincarnated into the noble Lamprogue family, he was presented to Count Blaise Lamprogue as the son of Blaise's missing younger brother, Gilbert, though publicly viewed as Blaise's illegitimate child. His stark black hair and eyes marked him as an anomaly.

His early childhood was isolated; family and servants shunned him, perceiving demonic traits and his status as a "concubine's son." A magic aptitude test at age three detected no mana, triggering the awakening of his past-life memories and personality. This halted his uncontrolled destructive outbursts but deepened familial estrangement. He dedicated himself to mastering onmyojutsu in this magic-dominated world, crafting tools like talismans (hitogata) from self-made paper at seven and creating shikigami from leaves at six due to paper scarcity.

His personality reflects caution forged by betrayal. He maintains emotional distance, distrusting all except Yifa, his servant and first friend. Typically calm during crises like monster attacks or assaults, he exhibits dissonant serenity when eliminating threats. Rare losses of composure occur during carriage travel due to severe motion sickness or when allies like Yifa face danger. His core drive is securing personal happiness through cunning calculation, avoiding past mistakes of overt power displays. He actively supports Amyu, the prophesied Hero, intending to live peacefully under her legacy while manipulating events unseen.

His abilities center on inherited spiritual power from his past life, compensating for zero mana. As an exorcist, he commands shikigami, ayakashi summons, curses, and barrier techniques. He manipulates the five Taoist elements alongside Yin and Yang energies, avoiding public use of wood and metal elements to prevent detection. Versatile spells include "Oni Flames" (fire/earth), "Mercury Binding" (wood/metal), "Fall of the Sky" (yang), and "White Blaze" (fire/yang), often channeled through talismans. Status assessments label him "Human/Divine Demon" with max-level skills in sorcery, exorcism, and divination, alongside extraordinary MP reserves. His lone physical weakness is motion sickness during vehicular travel.

Relationships hinge on selective loyalty. Yifa, his elven-descended servant, receives affectionate head pats and familial protection, though he remains oblivious to her romantic feelings. His brother Grey initially bullied him but developed fearful respect after confronting Seika's power. Amyu, the Hero, benefits from his protection; yet his intervention inadvertently stalls her growth, drawing political scrutiny. The ayakashi Yuki serves as a constant fox-formed companion. Fiona Urd Alegreif, a princess with foresight, mistakenly identifies him as the prophesied Demon King, creating tension. Broader connections include confronting Mabel Crane to honor her brother's memory and later joining Sora's adventuring team.

His development reveals unintended consequences. His identity as the Demon King emerges through others' perspectives, implying his soul occupied a vessel meant for that entity. Protective actions toward Amyu disrupt her destined path, leading factions to deem her a liability. Despite seeking anonymity, his power repeatedly forces conflict, balancing ruthless threat elimination against a persistent desire for quiet fulfillment. This duality defines his journey: immense capability constrained by self-imposed restraint, yet inevitably shaping the world's fate.