Rui

Rui was a member of the Twelve Demon Moons, having held the position of Lower Moon Five.

Appearance
Rui has the appearance of a young boy. His skin is white, with red dots. His hair is light pink and bears resemblance to spider legs. The sclera of his eyes are red, while his irises are pale blue. Due to holding the position of Lower Moon Five, Rui has the Kanji of five, tattooed into his right eye signifying his rank.

As a human, he possessed normal black hair styled with the same spider-leg like haircut, with bright blue eyes.

Personality
Rui was a disturbed individual who desired the close bonds that a family has and expected family members to perform their own roles. To him, "family members" who do not perform their roles as he expects them to (namely to protect Rui unconditionally or follow his instructions) are worthless and as good as dead.

Despite the fact he yearns for a "family" his vision of one is incredibly twisted and founded on nothing but fear of him, instilled through repeated abuse on his fellow kin, which he does with unrepentant sadism.

Well aware of his power, he is incredibly authoritarian and cruel and has no qualms about mistreating others or punishing his "family" by stringing up them up and left to be burned by the sun when they go against his wishes. Because of his strength, he tends to act almighty and confident, although this arrogance while hardly unfounded, does not blind him to potential threats to his life.

Interestingly Rui is incredibly sensitive to the subject of family and was furious when Tanjiro pointed out the more than evident toxicity of the bonds forged between his "family",this angered him to the point that he demanded that Tanjiro retract himself and his words and threatened to kill him if he did not comply. This stance contrasts heavily with his desire to have Nezuko for himself as his sister, after witnessing her selfless act of shielding Tanjiro from his threads, becoming obsessed with the idea of developing a bond with her, albeit still through fear and torture. This contrarian behavior is further evidence to Rui's delusions, mental insanity, and emotional instability.

Demon Abilities
Due to being a demon and being one of the Twelve Demon Moons receiving more blood from Muzan Kibutsuji that then further augmented his power, Rui possesses incredible amounts of power, easily surpassing that of other demons, seeing as how he managed to conquer and coheres numerous demons in Natagumo Mountain and force them to work for him and become his family due to them recognizing that he far outclassed them. Evidence of his strength is further shown when he was able to easily overwhelm talented fighters like Tanjiro Kamado and kill scores of demon slayer with little effort. Blood Demon Technique: When Rui uses his full strength, he begins pumping more of his blood into his hands that then extend into his threads, increasing the threads sharpness and cutting ability of the threads to then perform his Techniques.
 * Enhanced Durability - Rui has remarked that his body is incredibly durable and hard to cut, with it easily far surpassing the hardness of lesser Demons, as Tanjiro noted that Rui skin was much thicker that of his "Father", Rui even admitted that the durability of his skin far surpassed the durability of his own threads.
 * Enhanced Strength - Rui has displayed superhuman strength, being able to easily physically overpower Tanjiro by simply kicking him, despite he himself also possessing enhanced physical strength.
 * Enhanced Regeneration- Being one of the Twelve Demon Moons Rui possess powerful regenerative power, as seen when he easily healed and showed no visible pain at having his face slashed and clawed by Nezuko and later was able to reattach his head after decapitating himself.
 * Threads - Much like the rest of his "family", Rui main form of combat uses threads that he creates using his own cells for offense and defense, with the sharpness and durability of the threads being powerful enough to easily break a Nichirin Blade and being able to easily subdue and capture fellow Demons.
 * Demon Blood - Similar to Muzan, Rui can augment and strengthen a fellow demon's overall power and give them access to new abilities by giving him a drop of his own blood, having done so in order to strengthen each and every one of his "family" who were all originally weak demons.
 * Cell Manipulation - As revealed by the Older Sister Spider Demon, Rui has the ability to manipulate and change the physical appearance of other demons who consume his blood, having altered the physical traits of his "Sister" to match his own,he also likely did the same to his other "family" members to resemble him.
 * Cruel String Prison - Rui creates a web-shaped string around his opponent to cut him to pieces.
 * Cage of Murdering Eyes - Rui creates a cage from his web around the target, that continuously shrink until cuts the target to pieces.
 * Wheel of Chopping Threads - Creating a rotating wheel of threads, that upon reaching the opponent will cut him to pieces.

Human Life
Many years ago, Rui was born with a weak, frail and easily stressed human body that even the very act of walking and breathing was difficult for him to do, causing him to spend a majority of his life trapped within his home being cared for by his mother and father who despite his frailty deeply loved and cherished him.

One night, Rui was approached by Muzan Kibutsuji who then offered to "save" him and transformed him into a demon. Now possessing a strong body Rui was finally able to move about freely for the first time bringing joy and happiness to his parents. However, this did not last, as he was now a demon Rui now needed to obtain sustenance by eating humans which then caused him to kill a man inside his home. This bloody scene was then discovered by his parents who were immediately horrified by what their son had done. That night his father and mother agreed that Rui had to be stopped and attempted to kill him in his sleep to prevent him from doing anything worse in the future. His father then attempted to sneak up on him with a knife and told him not to worry for he planned to also kill himself in turn for killing his son, while his mother was sobbing and crying outside his door. However Rui awoke and in anger for their attempt on his life, killed them both instead in self-defense, rationalizing that his true parents would never try and kill him and thus considered them to be faked in denial. However, as his mother lay dying Rui overheard her regretfully apologizing to Rui for not being able to give him a healthy body as she died, which then caused him to remember his father last words, promising that both he and his wife would follow Rui to the afterlife by killing themselves, caused him to realize that his parents did indeed love him and simply tried to stop him from fully becoming a monster, throwing Rui further into emotional turmoil. However, Rui persisted on the lie that his bond with his family was fake due to receiving words of encouragement from Muzan, who supported his initial denial that they couldn't possibly be his family due to not accepting his state as a demon, and gave him encouragement to simply look for a new family.

Demon Life
Now as a Demon Rui continued to kill and eat human gain more strength until he finally obtain the position of Lower Moon Five and joining the Twelve Demon Moons after being acknowledged by Muzan. Due to repressing his painful memories as human, Rui requested to Muzan if he could create a "family" for himself the latter then allowed him and he began to gather suitable "family members" who would each take the role of his new parents and siblings.

In the Anime it was shown that he stumbled upon a lone female demon while she was fleeing from a group of Demon Slayers, he then simply asked her if she wanted to be saved by him and if she agrees she'd have to do everything he says, desperate for her life, the Demon agreed, Rui thus then easily dispatched the group of demon slayer easily and warmly welcomed the Female into his "family". He then proceeded to give her a small drop of his blood to strengthen her power and painfully alter he physical appearance to resemble his own and officially acknowledged her as his new "Older Sister". Later Rui was informed by the Older Sister Spider Demon about how one of his other older sisters was planning to escape the mountain and leave the family, enraged by this he proceeded to intercept this attempted escape and violently beated the Oldest Sister Spider Demon before stringing her up and leaving her to burn by sunlight of morning.

Trivia

 * Rui was ranked in 16th place as of the first character popularity poll with 191 votes.
 * Rui is one of two members of the Twelve Demon Moons who met the people he knew in life while disappearing; the other being Akaza.

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Руи