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 the left side of her hair, a 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 Lye Batenkaitos, erasing her existence from the world and putting her in suspended animation.
Rem lived 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, who believe in individual strength above all else, and were despised for it. They were to be killed as part of the custom of Oni to protect their pride as the strongest of the demi-humans. However, just before the execution, Ram displayed the potential of her power to protect them both, which shocked the community. Being allowed to live thanks to her older sister, who had an aptitude that had not been seen in a millennium among Oni kind, Rem spent the rest of her days in her shadow. Despite the presence of her sister also raising expectations for herself in the community, she could never meet them or surpass her sister in any area, which earned her disappointment from the other villagers. However, Rem loved her sister and accepted her place behind her and her parents still cared for them both deeply, even if she herself found it difficult to believe in front of the brilliance of her older sister. One night Rem's life changed forever when the village was attacked by the Witch Cult around 6 years before the beginning of the story, during which her sister's horn was cut off by Faust. The whole village got destroyed and her parents and the other citizens were killed, except for her and Ram.
Ever since then, Rem's inferiority complex had deepened due to a sense of guilt for momentarily feeling joy for 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.
- Healing Magic: Rem summons water mana to heal the wounds of herself and others, however it only affects physical ones. Power varies by the skill and talent of the individual or partially that of spirit if the caster is using Spirit Arts.
- 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 spears from her hands which easily pierces through and crushes flesh that vanish a moment afterwards.
- 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, though since she only has one, she is physically weaker than the average oni and tries to make up for it through the use of magic and weaponry.
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.
- Rem 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 of the rest of it.