Rui Arneb

Louis 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. She reappears in Arc 7 as a supporting character.

She is a spiritual being, born without a body. Learning of Return by Death, she ate Natsuki Subaru's memories and possessed him using a copy of herself in order to gain his ability. However, Subaru ultimately managed to defeat her and left her for dead, though she is later revealed to have survived but now has amnesia herself. She was transported along with Rem and Subaru to Vollachia.

Appearance
Louis has large blue eyes and extremely long blonde hair that can cover the floor like a golden sea. Her body is white as pottery and she wears rags unbefitting her beauty. She is physically in her mid-teens, though her exact age is unknown. During the events of the 6th Arc in the Corridor of Memories, she made her appearance in an elegant and almost transparent white dress which suited her extremely well.

As Louis is a spiritual being without a body that has been trapped inside the Corridor of Memories since her birth, she has occasionally possessed the bodies of her two brothers Lye and Roy 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 of a lot of peoples' lives. She is also described to be evil intent in the shape of a child. Louis doesn't care much about what she eats, putting more emphasis on who she eats with. Also, the three speak in a collective manner using us and we instead of first or third person.

Being the youngest sibling, Louis initially​ gives 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. Louis believes that life is unfair and is like a conveyor belt, without having the ability to choose the family, environment and future to be born into. Advocating her actions and beliefs as a way to search for happiness, she mentions that she is making an effort to find the life that will satisfy her and let her live with her head held high, and currently believes that Subaru's​ life will provide her that fulfillment, as he is able to return by death, which will give her the ability to undo any mistakes she may make in future.

After experiencing Return By Death, the Louis that had entered Subaru's soul as a Witch Factor became traumatized and got into a fight with the Louis within the Corridor of Memories, who retained her original personality. The end result of this confrontation was that Louis became mind-broken, regressing into an infant-like state.

After mysteriously materializing outside of the Corridor of Memories, Louis'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 had openly abused her, Louis continues to act friendly towards him.

History
Currently, not much is known about Louis's past other than she was born without a body, has been confined to the Corridor of Memories since her birth, that she is the younger sister of Lye Batenkaitos and Roy Alphard, and that she is one of the three Sin Archbishops of Gluttony, being the one that represents the aspect of Satiation. Like her two older brothers, it is unknown how long Louis has been a Sin Archbishop as well as both how and when she received the Witch Factor of Gluttony.

During the events at the Pleiades Watchtower, she lost her brothers with Lye Batenkaitos being killed and Roy Alphard being captured. After the tower was recaptured, she, along with Subaru and Rem, were sent to the Empire of Vollachia by Satella. Later, it is revealed that Louis does not smell of the Witch's Miasma, making it unknown if she retains her Authority after obtaining a physical form.

Abilities
Authority of Gluttony (暴食の権能 Bōshoku no Ken'nō): Louis is able to use the Authority of Gluttony. Master Class Fighter: Through the accumulation of souls eaten, Louis gains any abilities her victims once had such as martial art techniques and magic. In turn, this has granted her extraordinary combat strength. Splitting: Similar to the Oousagi, Louis can split and create a copy of herself whenever she separates her Witch Factor of Gluttony into multiple pieces. She originally demonstrated this ability when she created a copy of Lye while possessing him during Arc 5. A copy that she creates can possess someone else as shown with Subaru in Arc 6, while her original self remains inside the Corridor of Memories. Louis can split and create more than one copy of herself, but it is not known if there are limitations in the amount of copies that she can create.
 * Gluttony (暴食 Bōshoku): Louis can eat the name and memories of their 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 Louis, 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.
 * They 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 the 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 the Leaper technique. It is worth noting that by eating a person's name and using Lunar Eclipse, Louis does not gain her victims' Divine Protections or Spirit Arts.
 * Solar Eclipse allows the holder to harvest his victims' name and transform their appearance accordingly. Louis, 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, Louis tends to primarily use Solar Eclipse as she doesn't share the same fear as her two brothers.
 * Demon Beast Control: Because Lye, Roy and Louis possess the Witch Factors of Gluttony which were previously in Daphne's possession, they can direct Demon Beasts to attack certain areas. This is why the White Whale’s attacks were coordinated with those of the Witch Cult. Despite her ability to control the White Whale and the Great Rabbit, even if Louis had help from her older brothers, they wouldn't be capable of controlling the Black Snake, according to the author.
 * Leaper technique: This technique allows Louis to move with extreme speed undetectable by the naked eye, essentially allowing her to leap behind his opponents or avoid extremely precise and quick attacks. Due to the extreme speed, it seems like she's teleporting around the battlefield. She's also capable of utilizing it vertically, making it appear as if she's flying. This move originally belonged to Dorkell.
 * Carnivorous Beast: In the Web Novel, Louis displays this ability which allows her to strengthen her body which gives her two advantages: she can withstand powerful attacks and 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 and deliver a retaliating punch that is said to be as devastating as getting hit by a car. This ability formerly belonged to Beli Hainelga.

Trivia

 * Like the other Sin Archbishops (with the exception of Stride Vollachia), Louis 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 Louis Arneb, Roy Alphard and Lye Batenkaitos, were to fight, it would most likely end in a draw.