Rui Arneb

Rui Arneb (ルイ・アルネブ) is a Sin Archbishop of the Witch Cult, representing the Satiation aspect of Gluttony. First introduced in Arc 5, she later served as one of the main antagonists of Arc 6, alongside her brothers, Lye and Roy. She later becomes a supporting character in Arc 7.

After a chance encounter with Natsuki Subaru, she learned of Return By Death and attempted to steal his existence. However, Subaru ultimately managed to defeat her and left her for dead during the events of Arc 6, though she is later revealed to have survived at the cost of amnesia herself; bearing a completely new personality. She was transported along with Rem and Subaru to Vollachia at the end of Arc 6. Despite seemingly losing her original personality, she retains her Authority.

Appearance
In contrast to her brothers, Rui has large blue eyes, light skin, and extremely long blonde hair that can cover the floor like a golden sea. However, as is the case with Lye and Roy, she has fang-shaped teeth which help her smile look to be one more reminiscent of a menacing smirk. She normally wears an elegant white dress that nearly looks transparent. It comes with a jabot that has two small red lines along the leftmost side. She is physically in her mid-teens, although her exact age is unknown.

Rui's debut in Arc 5 featured her in her brother Lye's black cloak. It was tied just below her neck and it had uneven tears along the lower portion. As Rui's physical self had been trapped within the Corridor of Memories until Arc 6's climax, she occasionally took over the bodies of her brothers in order to experience the outside world.

Personality
Subaru describes her as a girl that'd look good with a smile, yet he describes her "smile" as a smirk, and gave off the feeling that she made light on the lives of many people. She is also described to be evil intent in the shape of a child. Rui doesn't care much about what she eats, putting more emphasis on who she wants to eat, befitting of her role as Satiation. Also, the three speak in a collective manner using us and we instead of first or third person.

Being the youngest sibling, Rui initially​ gave off the impression of being respectful and loving towards her brothers, but later reveals that she found them idiotic for blindly eating lives for her just because she was unable to exit the Corridor of Memories. Rui believed that life was unfair in a manner similar to that of a conveyor belt, without having the ability to choose the family, environment, and future to be born into. Because of these beliefs, Rui developed a goal of searching for, devouring, and becoming someone else all in the name of happiness, which would have allowed her to live with her head held high in a life she considers "ideal".

In her desire to achieve a happy life for herself, Rui was very shrewd, claiming that Subaru helped his friends out of pity and that he did it all for his own gratification as part of her plan to get him to reject the existence of the ‘Natsuki Subaru’ and start thinking of himself as a separate individual from the original him. When she experienced Return by Death, the Rui that had entered Subaru as a Witch Factor became traumatized and got into a fight with the Rui within the Corridor of Memories, who retained her original personality. The end result of this confrontation was that Rui lost her original mind, regressing into an infant-like state.

After mysteriously materializing outside of the Corridor of Memories, Rui's mind deteriorated into an infantile state. She is currently incapable of speech and complex thinking, instead displaying a mind driven by instinct similar to an animal's. Interestingly, she seems to have an amicable inclination towards Subaru in this state, despite his previous actions against her. She would even go as far as to affectionately lick his face and protect him when others threatened his life, as was the case when she tackled Rem when the demon girl tried to strangle him. Even after Subaru treats her badly, Rui continues to act friendly towards him.

History
Since her birth, Rui was confined in the Corridor of Memories. Despite being the younger sister of Lye Batenkaitos and Roy Alphard, they were unaware of her existence. Due to her confinement, she only found satisfaction in the occasional "meals" that she was able to receive from her brothers which came from the effects of the Authority of Gluttony. Later, she found a way to temporarily escape by controlling the bodies of her brothers, which made them aware of her existence. She later gained access to Eclipse after her brothers discovered it and used it to devour people in her search for happiness.

Abilities
Authority of Gluttony (暴食の権能 Bōshoku no Ken'nō): Rui is able to use the Authority of Gluttony.
 * Gluttony (暴食 Bōshoku): Rui can eat the name and memories of an opponent. To eat a name, she touches them with her left hand, then licks the palm of the same hand. Things such as experience, talent, and the likes and dislikes of human relationships serve as food for her, allowing her to take advantage of any abilities or memories her opponents had. However, if she eats a fake name, she becomes plagued with severe nausea. Even after killing Rui, the names and memories of the forgotten will not return to their original owners.
 * Those that have had their names eaten have their existences erased from the world, destroying any human relationships along with negating any contracts with Spirits.
 * Those that have their memories eaten become amnesic and any characteristics they may have had are reset, effectively making them into a different person.
 * Those that have both their name and memories eaten simply become a shell, and although they live, they no longer eat, excrete waste, or age, putting them in suspended animation.
 * She cannot eat the same person twice until that individual gains enough new memories to establish a different sense of self from their old one.
 * Eclipse (蝕): Eclipse is considered to be the trump card of Gluttony. It is divided into two types; Solar Eclipse (日食 Nisshoku) and Lunar Eclipse (月食 Gesshoku).
 * Lunar Eclipse allows the holder to use their victims' memories and harvest their skills and knowledge. Lye, for example, used Lunar Eclipse to greatly boost his physical strength and perform numerous martial art techniques, including the Palm of the Fist King and Leaper techniques. It is worth noting that by eating a person's name and using Lunar Eclipse, Rui does not gain her victims' Divine Protections or Spirit Arts.
 * Solar Eclipse allows the holder to harvest their victims' name and transform their appearance accordingly. Rui, for example, transformed into Neiji Rockheart and numerous other victims she and her two brothers had eaten throughout the years, using her sibling's body. Both Lye and Roy dislike having Solar Eclipse used on them. According to them, Solar Eclipse made them feel as if their selves were being damaged, making it unendurable. Unlike Lye and Roy, Rui tends to primarily use Solar Eclipse as she doesn't share the same fear as her two brothers.
 * Demon Beast Control: Because Rui, Lye, and Roy each possess the Witch Factor of Gluttony which was previously in Daphne's possession, they can direct Demon Beasts to attack certain areas. This is why the Hakugei's attacks were coordinated with those of the Witch Cult. Despite her ability to control the Hakugei and the Oousagi, even if Rui had help from her older brothers, they wouldn't be capable of controlling the Kurohebi, according to the author.
 * Creation: Rui stated she and her brothers can create life with the usage of her Witch Factor of Gluttony, just like how Daphne created the Demon Beasts four centuries ago. However, Rui has also stated she and her brothers had no reason to do so, hence they never followed through the idea.

Master Class Fighter: Through the accumulation of souls eaten with her Authority, Rui gains any abilities her victims once had such as martial art techniques and magic by using Lunar Eclipse. In turn, this grants her extraordinary combat strength.
 * Leaper (跳躍者): This technique allows Rui to use a short-range teleportation in quick succession, essentially allowing her to "leap" behind her opponents or avoid extremely precise and quick attacks. She is also capable of utilizing it vertically, making it appear as if she's flying. This ability originally belonged to Dorkell.
 * Carnivorous Beast (肉食獣): In the Web Novel, Rui displays the aforementioned ability which allows her to strengthen her body which gives her two advantages: She can withstand powerful attacks, and she can strengthen her limbs to deliver devastating blows. With this ability, she was able take a hit from Emilia's ice spear and came out unscathed. She was also able to deliver a retaliatory punch that was said to be as devastating as getting hit by a car. This ability originally belonged to Beli Hainelga.
 * Unnamed Earth Throwing Technique: In Arc 7, Rui utilizes an unnamed earth throwing technique to repel a group of horned demi-humans. When peeling off chunks of nearby walls or concrete, it’s mentioned that the earth takes on a softer texture and is peeled away as easily as lifting up a carpet. When thrown, it regains its usual hardness.

Splitting: Similarly to the Oousagi, Rui can split and create a copy of herself whenever she separates her Witch Factor of Gluttony into multiple pieces. With a copy, Rui can not only attach herself to someone else as a Witch Factor. In Arc 6, Rui used this ability to attach a copy of herself to Subaru, which allowed her to forcibly trap his memories up to that point within the Corridor of Memories. She also had the ability to take control of someone for a brief period of time, which she occasionally used to possess the amnesiac Subaru so she could carve into his arm or commit actions like murder, thereby instilling distrust and paranoia within him. Rui can split and create at least two copies of herself, but it is not known if there are limitations in the amount of copies that she can create.

Trivia

 * Like the other Sin Archbishops (with the exception of Stride Vollachia), Rui received her namesake from a celestial body. "Arneb" was a former name for Alpha Leporis, the brightest star in the Lepus constellation. The word was derived from the Arabic term أرنب (arnab), which translates to "Hare".
 * In a Q&A, the author stated that if Daphne and a Sin Archbishop of Gluttony were to fight, it would most likely end in a draw. It is unknown if this answer was stating that a fight with Rui and her siblings would end in a draw against the Witch of Gluttony if they fought her all at once, nor is it known if the resulting draw would only come from a match with one of them.
 * In Arc 6, Rui has her own named chapter in both the Web Novel and the Light Novel.