Haruyoshi Kuga was once the most powerful onmyouji, or exorcist, in his original world, active during the Heian period of Japan. Despite his immense power, which he used to protect the Imperial Court, his strength bred fear and jealousy among the nobility and even his own peers. This ultimately led to his betrayal, and he was killed by his own disciple at the behest of those who envied him. With his dying breath, Haruyoshi used a secret, self-created curse to reincarnate, swearing to himself that in his next life, he would prioritize cunning over raw power to secure a quiet and happy existence, avoiding the pitfalls that led to his downfall.

He is reborn as Seika Lamprogue, the illegitimate and seemingly magicless third son of a noble family in a European-style fantasy world. True to his past self, Seika retains all his memories and the spiritual power he commanded as Haruyoshi, though he possesses no mana, the source of magic in his new world. The personality of the reincarnated Seika is defined by the lessons learned from his tragic first life. He is a largely stoic and calculating individual who deliberately hides his true abilities to avoid attracting unwanted attention. Having been betrayed for being too powerful, he adopts a low-profile, cunning approach, only using his exorcist arts when necessary and ensuring that he is underestimated by those around him. While his calm demeanor is his default state, he does have a quiet, mischievous side that emerges in his schemes and interactions, and he experiences notable discomfort from mundane issues like motion sickness. His primary motivation is to achieve a peaceful, happy, and unassuming life, a simple goal he was denied in his past existence.

In the story, Seika’s role is that of a hidden prodigy who navigates a world that views him as a failure. He enrolls in the Lodonea Academy to discreetly support and mentor Amyu, a young woman prophesied to be the Hero destined to defeat the Demon King, hoping to live safely under her protection. His journey is heavily influenced by the relationships he forges. A key figure is Yifa (also known as Efa), a servant and his first true friend in his new life. She is fiercely loyal and capable of communing with spirits; Seika trains her to control her power and comes to view her as family. His relationship with Amyu, the Hero, is complex. He pledges to make her the world’s strongest warrior, but his plan is upended by the revelation that his soul has reincarnated into a vessel meant to become the new Demon King, creating a profound tension between his role as protector and his destined identity as the enemy. He also forms a pact with Yuki, a shapeshifting fox spirit ayakashi whom he summoned and who serves as a loyal familiar.

Seika’s character development is a journey from isolating himself to protect his peace, to forming genuine, protective bonds that he values above his own goal of a quiet life. Despite his ambition to be cunning and detached, he repeatedly uses his immense power to save his friends, which consistently draws the attention he seeks to avoid. His arc culminates in his acceptance of his role as a protector who wields his strength for others rather than for dominance. His notable abilities are derived from the art of onmyoudou, rather than the local magic system. This includes manipulating his ki, or life force, to perform feats of exorcism, elemental control over fire, earth, water, wood, metal, yin, and yang, and summoning shikigami (familiar spirits) such as Yuki. He uses paper talismans, also known as hitogata, for a wide variety of purposes, including creating barriers, casting curses, healing, and even reflecting curses back upon their caster. He also possesses sensory abilities like "Dragon Vein Vision," allowing him to see the flow of spiritual energy, and can transform into an ayakashi form to access a greater level of power.