Lay Glanzudlii is the reincarnation of Kanon, the legendary human hero who lived two millennia before the main events. As Kanon, he brokered peace by allying with the demon king Anos Voldigoad, sacrificing himself to end the ancient human-demon war. This past directly shapes his reincarnated life, where he assumes the identity of a pureblood demon despite his heroic human origins. Reborn into the prestigious magic-wielding Yvesta family, he inherits no family magic due to interference from Syrica—a source magic within him tied to his existence as Kanon. Consequently, his family abandons him in early childhood.

Facing death at age five, Lay is rescued by Sheila, a half-demon, half-spirit woman who becomes his adoptive mother. She flees with him to escape persecution from her own family, the Yesla. Under Sheila's care, Lay cultivates extraordinary swordsmanship, rapidly mastering local dojos and dominating tournaments. His unmatched skill earns him the title "Demon Swordmaster" and entry into the Demon King Academy. During this time, Sheila contracts spirit disease and is hospitalized; Lay frequently visits her while hiding his true capabilities.

Physically, Lay possesses shoulder-length white hair and blue eyes. He typically wears a modified Demon King Academy uniform in black and red tones, featuring a long coat, belt, and heptagram badge. His personality defies typical demon nobility; he exhibits a mellow, friendly demeanor and rejects royalist doctrine while remaining neutral toward factions like the Unitarians. He treats all individuals equally regardless of heritage, embodying egalitarian ideals. Lay's primary passion is swordsmanship, though his innate talent initially led to boredom. This changed upon encountering Anos, whose challenge reignited his combat enthusiasm.

Lay's abilities are multifaceted. He wields immense magic power, physical strength, and speed, enabling him to rival Anos and swiftly overpower adversaries like the Elder Demon Emperors. As Kanon's reincarnation, he retains mastery over source magic, possessing seven distinct sources that allow self-resurrection if at least one remains intact. He can separate and fuse these sources with others, as demonstrated with the Seven Elder Demon Emperors. His swordsmanship transcends conventional limits; he skillfully employs demon swords, holy swords, and spirit swords simultaneously, combining their auras for enhanced effects. He rapidly assimilates opponents' techniques mid-battle and achieves pinpoint precision. Initially, he feigns magic ineptitude to mask his holy powers—a ruse to conceal his true identity as Kanon.

Key relationships include his cherished adoptive mother Sheila and his romantic partner Misa Ilioroagu. His dynamic with Anos evolves from respectful rivalry to profound camaraderie rooted in their shared history. Lay's loyalty drives him to protect Anos and their era's peace, even at the cost of his life. A significant consequence of his actions is the unintended creation of the real Avos Dilhevia—a spirit manifestation born from the fictional demon king identity he fabricated. This entity later merges with Misa's spiritual essence, complicating his emotional journey.

Throughout his development, Lay grapples with the burdens of his past life while navigating his demon existence. He reconciles his desire for peace with the sacrifices it demands, ultimately choosing to safeguard the world Anos helped build. His journey underscores themes of identity, sacrifice, and the transcendence of inherited conflicts.