Setsu Suzaki, the hero who saved Eclair in its previous war, was betrayed by the Human King of Distinia after his victory. Fearing Setsu's power, the king used transfer magic to forcibly reincarnate him as a baby on Earth, stripping his original appearance while preserving his memories and abilities. Reborn as Yuki, he lived sixteen years as a seemingly ordinary yet socially withdrawn high school student in modern Japan. His persistent research into interdimensional travel earned him a reputation as a gloomy bookworm, all while maintaining a low profile and secretly seeking a way back to Eclair.

His second summoning occurs unexpectedly when the Kingdom of Distinia summons him alongside his classmates to confront a renewed demon invasion. Transported back to Eclair five years after his initial departure, he finds the world embroiled in another conflict. His changed appearance prevents immediate recognition by former allies, allowing him to operate discreetly. Initially reluctant to engage due to his traumatic betrayal, he prioritizes reuniting with trusted non-human allies like Demon Lord Dezastol Serreno and the Beast King over aiding the human kingdom that betrayed him.

Personality-wise, Setsu exhibits a consistently calm and composed demeanor. He values loyalty and remains grounded despite his legendary status in Eclair, strategically leveraging his reputation only when necessary, such as mobilizing allies or navigating political tensions. His approach to conflict is pragmatic and efficiency-oriented, often masking his true capabilities to avoid drawing undue attention.

Setsu retains the combat skills and magical prowess from his first summoning. His primary weapon is the sacred sword Gluttony (also called Kuromaru or Hungry Fangs), an amorphous-type holy sword capable of devouring curses, spatial matter, life force, or even memories. Wielding this weapon demands high costs; insufficiently "feeding" it risks catastrophic collateral damage, and it has consumed portions of Setsu's pre-summoning memories. His techniques include multiple forms of Hiken (Flying Blade Slash), such as Annihilation and Barrage, alongside Spatial Magic for teleportation via Gates. Notable abilities include Earth Eater, which consumes terrain and organic matter at the cost of his mana or memories, and Limit Break for enhanced combat. He also employs Body Strengthening, Flash movement, and Disappearance—a divine-erasure skill usable only after ascension. His battle strategy emphasizes precise, experience-driven decision-making, often identifying optimal actions instantaneously.

Relationships form a core aspect of his journey. On Earth, he shares a close bond with Yuuhi Hanabashira, a classmate summoned with him who harbors romantic feelings for him. In Eclair, his network includes Elka Verso, a Distinia soldier with a devoted and masochistic streak toward him; Leviathan, the ocean goddess who adopts a tsundere attitude about her affections; and Dezastol, the Demon Lord he befriended during the first war. He mentors figures like Loa Leoneir, daughter of the Beast King, and aids characters such as the beastgirl sisters Shironeko and Mineko, curing the latter of a curse. His central antagonist is Tōma, a fellow Earth-summoned individual from his first journey who orchestrated both wars to usurp godhood and eradicate all life.

Setsu conceals his identity as the original hero to most characters initially, driven by practical caution and a desire to avoid past pitfalls. His gradual disclosure to key allies enables coordinated efforts against Tōma's machinations, emphasizing trust built through shared history or proven loyalty.