Natsuki Subaru is a teenage boy with spiky black hair, brown eyes, and an athletic build, often clad in a tracksuit from his original world. Thrust from modern Japan into a fantasy realm, he wields the enigmatic ability Return by Death, rewinding time to a prior checkpoint upon dying. This power binds him to secrecy—disclosure risks violent silencing, endangers allies, or summons hostile entities like Hamsuke and Death Knight, drawn by the Witch’s scent lingering on him. While Return by Death functions in the Isekai Quartet crossover world, its capacity to revive him after permanent demise remains uncertain. Optimistic and resilient despite enduring repeated trauma, Subaru channels his hardships into loyalty toward allies such as Emilia, whom he serves as a knight, and Rem, whose unwavering devotion once pulled him from despair. In Isekai Quartet’s school setting, he navigates camaraderie with peers like Ainz Ooal Gown, overcoming initial unease toward the skeletal overlord, and shifts from rivalry to teamwork with Kazuma Satou, a fellow former shut-in. His tactical instincts surface during collaborative trials, exemplified by orchestrating defenses against the Mobile Fortress Destroyer during a field day. Subaru exhibits unique resistances, shrugging off high-tier magic like Ainz’s Time Stop and deflecting psychological tactics such as Tanya von Degurechaff’s intimidation. His humor emerges in comedic school mishaps, though he remains perplexed by Rem’s earnest affection and Darkness’s masochistic tendencies. A former hikikomori, Subaru’s insecurities stem from comparisons to his accomplished father, fueling his relentless drive to protect others. This resolve pushes him to endure agony to retain memories of his deaths. In Isekai Quartet, he adapts to the lighthearted school life without shedding core traits—strategizing alliances with Reinhard van Astrea’s class against mutual threats while balancing loyalty and wit. Key crossover moments involve probing his curse’s boundaries, forging temporary pacts during competitions, and facing existential risks like irreversible death. Despite the comedic backdrop, echoes of his original struggles linger, including battles against the Witch’s influence and efforts to bridge cultural divides in relationships.