Rem has medium length sky blue hair that covers her right eye, large light blue eyes, and young features. She also has hair clips towards left side of her hair, flower-shaped ribbon on the same side of her hair, and a maid hairband. She looks nearly identical to her sister apart from her hair, eyes, and ribbon coloring. She usually wears a maid uniform, and since her chest is slightly larger than her sister's, she secretly changes the measurements to hide it.
Rem tends to speak in a superficially polite manner and sometimes acts without thinking. She holds both respect to the point of worship and guilt toward her sister for an incident in the past. Because of her respect and guilt toward her sister, she used to think of herself as inferior. After Subaru saves her, she falls deeply in love with him, becoming one of his most loyal followers. Rem will stick with him no matter what happens, even if Subaru rejects himself.
Currently Rem has had her name and memories eaten by Ley Batenkaitos, erasing her existence from the world and putting her in suspended animation.
Rem lived along with her father, mother, and her elder sister Ram in an Oni village. Initially, because they were born with one horn each it meant they were defective in the view of the Oni community whom believe individual strength above all else and were despised for it. They were to be killed as the custom of Oni to protect their pride as strongest of the demi-humans, until just before the execution Ram displayed the potential of her power to protect them both at which shocked the community. Being allowed to live thanks to her older sister who had an aptitude that had not been seen in a millennia among Oni kind, Rem spent rest of her days in her shadow. Despite presence of her sister also raising expectation for herself in the community, she could never meet them nor surpass her sister in any area which earned her disappoint from the other villagers. However Rem loved her sister and accepted her place behind and very least her parents still cared for them both deeply even if she herself found difficult to believe in front of the brilliance of her older sister. One night Rem's life change forever when it got attacked by the Witch Cult around 6 years before the beginning of the story, following into her sister's horn being cut off by Faust. The whole village got destroyed along with her parents and the citizens, except for her and Ram.
Ever since, Rem's inferiority complex had deepen due to a sense of guilt for momentary feeling of joy her sister's injury that night, so she made the utmost effort to become a replacement for her sister as ideal a servant as possible to atone for what she felt had robbed from Ram with her. But even having reached a certain level of perfection, she still has a low level of self-esteem and continues to idealize Ram.
Magic User: Rem is skilled at Water Magic and Roswaal estimates her skill as being in the lower high. She is able to attack and heal using her magic.
- Huma (ヒューマ): Rem creates a shining water barrier that blocks other projectiles and sorts of magic. However, if the incoming magic is stronger than the shield it will get through it and cause damage to the user.
- El Huma (エル・ヒューマ): Rem forms and shoots multitudes of ice spikes from their hands which easily pierce through human flesh that immediately vanish on impact.
- Ul Huma (ウル・ヒューマ): Rem materializes and shoots three pillars of ice, each as thick as a hundred spears bundled together.
- Al Huma (アル・ヒューマ): Rem creates a sharp ten meter long spear of ice. It can also be modified to trap enemies. She can create up to 4 of them.
Synesthesia: An ability that she shares with her sister. It allows them to share their feelings with each other through telepathy
Oni Bloodline (鬼の血族 Oni no Ketsuzoku): As she is an Oni, Rem can enter her Oni Form, manifesting a single white horn on her forehead. The horn increases her physical abilities and acts as a gate that gathers mana from the atmosphere.
Equipment: Rem wields a Flail (Chain Morning star) in combat.
- According to the author, Rem's birthday is February 2, the same as her twin sister Ram.
- Likes to read in her spare time.
- She normally handles 80% of the workload in the mansion on her own. Subaru is usually paired up with Ram to take care the rest of it.