A sole survivor of the Witch Cult's massacre against her Oni village, where her parents perished and her twin sister Ram suffered horn loss, Rem was raised alongside Ram as a maid by Margrave Roswaal L. Mathers. She harbored a deep inferiority complex toward her prodigious sister, striving relentlessly to prove her worth by managing nearly 80% of the mansion's duties. Initially distrustful of Subaru Natsuki due to the Witch's scent clinging to him, Rem's perspective shifted when he saved her from demon beasts, igniting profound loyalty and unwavering love. She pledged eternal support to him regardless of circumstance. Accompanying Subaru to Lugunica's capital in Arc 3, Rem aided him against the Witch Cult's attack on the Mathers domain. During their return convoy, the Sin Archbishop of Gluttony, Lye Batenkaitos, assaulted her. Using his Authority, he consumed her name and memories, plunging her into a coma and erasing her existence from nearly all memories except Subaru's. She awoke at Arc 6's end afflicted by amnesia, devoid of name or recollection. Transported alongside Subaru and Rui Arneb to the Sacred Vollachia Empire, she remained without identity or past memories. Within the Isekai Quartet parallel world, Rem serves as school nurse alongside Shalltear Bloodfallen. Her devotion to Subaru persists; she expresses faith in his abilities and prioritizes his well-being, scheduling private physical examinations and reacting intensely to his injuries, demanding immediate medical attention. She physically carries him to school when tardy, showcasing her strength and singular focus on aiding him. Participating in school events like talent shows, field days, and study groups, she consistently encourages Subaru. Her Valentine's Day gift to him—a large heart-shaped chocolate box—contrasted with Emilia's smaller offering. Rem interacts cross-series, finding common ground with Albedo over shared romantic dedication. While on nursing duty, she calmly uses her flail's chain to restrain both Albedo and Shalltear when they attempt to abandon the office pursuing Ainz Ooal Gown. She pragmatically suggests they arrange private examinations for their own loved ones. Despite occasional misunderstandings, like Shalltear glancing at her chest during a beach trip, she maintains functional professional rapport with Shalltear. In *Isekai Quartet: The Movie – Another World*, a wormhole transports Rem to a golem facility. Joining Mare Bello Fiore, Aqua, Ram, and Naofumi Iwatani, she escapes a swarm of small golems, reaching a village. Later reuniting with transported classmates, she battles rampaging golems and contributes to neutralizing a massive golem threatening their return home. Rem's Oni physiology grants enhanced strength, speed, and durability, enabling effortless carrying of Subaru and wielding a morning star. She can manifest a single white horn on her forehead, exponentially amplifying physical prowess and mana absorption, though risking loss of control to berserk instincts. Specializing in water magic, she employs offensive and healing spells: Huma, El Huma, Ul Huma, and Al Huma. A synesthesia link connects her to Ram, allowing mutual emotional sensing. Within the spin-off *Pixel Game Maker Series ISEKAI QUARTET Adventure: Action Game*, Rem appears as support. She lends her abilities to assist playable characters like Albedo, Megumin, Emilia, Tanya, and Raphtalia when they gather specific in-game items.