Kelvin, a human from Earth, perished due to a deity’s miscalculation and was reborn in a new world after exchanging his memories for enhanced powers. Stripped of personal recollections but retaining modern knowledge and cultural references, he gained advanced summoning abilities alongside a blessing from the goddess Melfina, enabling monthly resurrection and the summoning of interdimensional heroes like Rion, a reincarnated girl he adopts as a sister.

Visually distinguished by black hair, a black battle robe with royal blue accents, metallic guards, and a sword-like accessory behind his left ear, Kelvin wields evolving weapons—from staff to scythe—earning the moniker "Grim Reaper." His race shifts from human to Daemon after pivotal battles, including the defeat of a Demon Lord.

A paradoxical blend of relaxed charm and combat obsession, Kelvin thrives on challenging foes, self-imposed handicaps, and tactical duels. He fiercely protects his companions, particularly women in his party, balancing flirtatious banter with unwavering loyalty and respect for their autonomy.

His S-rank Summoning skill permits contracts with ten followers, including the evolvable slime Clotho, undead knight Gerard, demon king’s daughter Sera, and Melfina—summonable fully only via an artificial vessel. Proficient in Green Magic (wind, earth) and White Magic (light, healing), he employs techniques like Boreas Death Scythe and Air Pressure, augmented by Magic Overclock and Parallel Processing for spell potency and multitasking. Smithing allows crafting high-tier gear, though early attempts yielded cursed items.

Past lives as an Angel betrothed to Melfina and a cursed Demon Lord slain by her resurface in fragmented memories, intertwining with residual Japanese cultural knowledge. Establishing a base in Parth, he forges alliances, battles adversaries like Demon Lord Gustav, and mentors heroes like Toya’s party through staged trials to spur growth.

Ethical ambiguities emerge in actions such as purchasing Efil, a half-elf slave, to break her curse—an act critiqued for implicit slavery acceptance. Anime adaptations omit romantic ties with Efil, depicting their bond as platonic.

Evolving from amnesiac adventurer to mythic leader, Kelvin navigates political intrigue, deepens familial bonds with Rion, and ascends as a Daemon, balancing combat fervor with protective stewardship over his allies and realm.