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 Rank 5, Rui has the kanji of Lower Rank (
As a human, he possessed normal black hair styled with the same spider-leg like haircut, with bright blue eyes.
On general basis, Rui is a quasi-calm individual, rarely raising his voice, and talking with everyone, demon or slayer, with a reasonable tones and vocabulary, though his low and guttural tones constantly imply repressed anger and annoyance. Every time he spoke with Tanjiro during their duel he remained somewhat civil in speech, never once insulting him or calling him a name.
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 them up and leaving them 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. Further, he is aware on some level that, despite telling Tanjiro when he saw Rui punishing Spider Sister that he wasn't watching a play, that his whole family is a sham, yelling at Spider Sister that none of his family member were able to fulfill their roles, or fill the void in his heart.
Because of this, Rui is incredibly sensitive to the subject of family and was furious when Tanjiro Kamado 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 Kamado 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.
In death, Rui did feel remorse for his actions, remembering a previous Spider Family member he had strung up to die in the sun because she attempted to leave him due to his ongoing abuse.
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.
Now as a Demon, Rui continued to kill and eat humans to gain more strength until he finally obtained the position of Lower Moon Five and joining the Twelve Kizuki 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.
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 which 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 alters her physical appearance to resemble his own and officially acknowledged her as his new "Older Sister".
Later Rui was informed by the Daughter 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 beat the Oldest Sister Spider Demon before stringing her up and leaving her to burn by sunlight of morning.
Rui first make an appearance in Episode 15, while Tanjiro, Murata, and Inosuke are fighting the Spider Demon Mother. He make an appearance by walking along a thread tied to tree tops, and announces that he will no let the slayers disturb his family's peaceful life. He then say Mother with make short work of the slayers. Inosuke then attacks him by jumping off one of the slayers but is still unable to get enough height. After which Rui just turns and walks off, with Inosuke wondering why he even showed up at all. Later, Rui is shown making a Cat's Cradle, saying he won't let anyone get in his family's way, and how they are going to live in happiness.
Later (next episode), Rui is seen approaching his "mother", which visibly terrifies her. He asks her if she can truly win. When her terror stops her from asking, he simply asks if the fight is taking to much time. He then says she better get to it, or he will tell father. Spider Mother promptly assures Rui she will protect him, begging him not to tell Spider Father. Rui stand for a minute expressionless and impassive, then turns and leaves, telling her to hurry it up.
Later, after a clash with the Spider Demon (Father), Tanjiro hears a scream. Investigating, Tanjiro hears more cries and pleas for mercy, and the sounds of someone being struck. It is revealed to be Spider Sister, on her knees clutching her face, injured, and with Rui standing over her. Rui doesn't move, but asks what Tanjiro is looking at, stating that it isn't a play. When asked if Rui and Spider Sister are on the same side, Rui tells Tanjiro not to phrase it in such an infantile manner. Rui goes on to say that they are family connected by a powerful bond, but regardless, it was between him and his sister, and that if Tanjiro interfered he would carve him up. At this point, Tanjiro called Rui and his family bonds a lie. Rui grew very angry at this, but was interrupted by another Demon Slayer. After shredding him Rui returns his attention to Tanjiro, demanding to know what was said to him.
When the scene returns, Rui has engaged Tanjiro, striking with his threads. He then states that he won't kill Tanjiro in a single strike, saying he will dissect him, unless Tanjiro retracts what he said, in which case he will make it quick. Later in the battle, after Tanjiro had developed the ability to detect his threads, Rui noted that Tanjiro was smarter than he gave credit for, and that he didn't shrink back in fear, but believed that it wouldn't matter in the end. A moment later, when Tanjiro attempt to strike aside a thread with the Water Breathing: First Form - Water Surface Slash, his sword however breaks the moment it makes contact with the thread.
Though cut, Tanjiro survives by dodging sideways. Rui continues with a patient assault, striking with his threads which Tanjiro continues to dodge with some difficulty. Tanjiro notes that Rui is holding back, but was still overpowering him. Rui breaks the silence by asking if Tanjiro refuses to take back what he said (about his familial bonds being fake). When Tanjiro doesn't respond, Rui desides that Tanjiro does refuse and to put and end to things. A large web descends on Tanjiro. Tanjiro, unable to dodge or block the strike, is protected by Nezuko.
While Tanjiro tends to Nezuko's injuries, the Spider Demon (Sister) speaks, noting how Nezuko emerged from her box to protect Tanjiro, despite being a demon. Shaking, Rui asks Tanjiro if Nezuko is his sibling. Tanjiro shouts at him. More to himself than anything, Rui being to note that Nezuko, despite being a demon, continues to stay with and protect Tanjiro at personal risk, noting how the bond is genuine. Rui then states that the bond will be his. Spider Sister then calls out to Rui, asking/begging him to reconsider, saying that she is his older sister and can't abandon her. Rui lashes out in anger, telling her to shut up as he slices her in four; at the neck, where the torso meets the abdomen, and at the waist. In a cold anger, Rui tells Spider Sister that none of them (the other members of the Spider Family) were able to perform the roles they were assigned. In response for her please for a second changer Rui orders her to kill the other humans wandering over the mountain. He says he will forgive her if she does that. Pulling herself together, she agrees and leaves.
Calling out to Tanjiro, Rui says he simply wishes to talk. Tanjiro, wary, confronts him. Rui tell Tanjiro that what Nezuko did moved him. He says that Tanjiro's fate is death, but that there is a way to escape that fate. He tells Tanjiro that if Tanjiro gives him Nezuko, he will spare Tanjiro's life. When Tanjiro expresses confusion over the fact, Rui clarifies by saying that Nezuko will his little sister instead of Tanjiro's. Outraged, Tanjiro angrily tells Rui that Nezuko has her own feelings and will never be his sister. Rui, unbothered, simply says that he is stronger than Tanjiro, and will create a bond with Nezuko, that she will know exactly what happens when he is defied. Infuriated, Tanjiro shouts that as long as Rui follows that mindset, he will never get what he wants. Unfazed, Rui simply asks that Tanjiro refrain from shouting, saying it's clear they don't agree. Stepping forward, Tanjiro declares he will never hand over his sister. Rui agrees, saying he will simply kill Tanjiro. Tanjiro responds by saying he will simply cut of Rui’s head first. Rui simply smiles in amusement, saying that is the correct spirit. He then challenges Tanjiro to try, revealing himself to be one of the Twelve Kizuki.
The next scene opens with Rui's narration, listing the roles of the various family members. He finishes this by saying that if someone doesn't understand their role, they shouldn't be alive to begin with. He then addresses Tanjiro, saying that his role is to hand over Nezuko and leave. Rui finishes by saying that noncompliance is death. Tanjiro doesn't back down, and simply looks for a path to victory. Angered, Rui shouts that Tanjiro cannot defeat him. Pulling back his arm, Rui grabs Nezuko. Rui declares the matter settled, Tanjiro attacks. Rui asks Tanjiro if he remembers that he said he would spare Tanjiro's life if he complied. Nezuko scratches out Rui's face, but he remains passive. Rui strikes at Tanjiro but he evades, but then notices that Nezuko is gone. When blood falls onto his sword, Rui reveals that he strung up Nezuko, and will have to discipline her, and if she doesn't learn she will be left to burn in the sun.
Angered, Tanjiro attacks but is quickly stopped by Rui's threads. Walking up to him, Rui kicks Tanjiro sending him a considerable distance. Rui then backfist's him. Feeling confident, Rui challenges Tanjiro to cut of his head, saying how his skin is stronger than his threads, which Tanjiro cannot cut. Nezuko begins to struggle, to which Rui tightens his threads, reminding her that he is now her elder brother and commands she cease. Moments later, Nezuko passes out, to which Rui notes she is unlike any demon he has ever encountered. Again, Tanjiro attacks to which Rui counter attacks, asking if he can see that it is futile, to which Tanjiro uses the Total Concentration: Water Breathing: Tenth Form - Constant Flux to cut Rui's threads, something he had been unable to do up to that point. Noting that Tanjiro's new method was a potential danger, Rui strengthened his threads, saying farewell as he unleashes his Blood Demon Art: Cutting Thread Cage. In a moment of desperation and near death, Tanjiro, for the first time ever in battle, uses the Hinokami Kagura, and is able to slice through Rui's reinforced threads using Dance. With his new strength, Tanjiro presses the attack, and is able to get the upper hand against Rui. Awakening, Nezuko helps her brother, using and unlocking her Blood Demon Art: Exploding Blood, both burning Rui, his threads, and enhancing Tanjiro's sword, to the point he is able to behead Rui.
On the ground, Tanjiro, exhausted, tries to crawl over to Nezuko, so they can save Inosuke when he smells the scent of blood. Rui reveals himself to be alive, having beheaded himself to avoid Tanjiro's sword. Rui announces that enough is enough and that he will simply kill Tanjiro and Nezuko, and that he hasn't been this angry in a very long time. Walking over, Rui wonders aloud about Nezuko's power, and draws his threads. As Rui uses his Blood Demon Art: Murderous Eye Basket, Giyu arrives and destroys the technique. Announcing that he will finish the fight Giyu faces off against the Lower Rank.
Calling the Demon Slayers a torrent of insignificant insects, Rui attacks his most powerful Blood Demon Art: Cutting Thread Rotation. Giyu responds to the attack with Total Concentration: Water Breathing: Eleventh Form - Dead Calm. Rui angrily shouts that it doesn't matter what the technique is when his own assault is immediately defeated. Shocked and confused Rui moves to perform the attack again, but Giyu is gone. A moment later, Rui is decapitated.
Overall Abilities: Due to being one of the Twelve Kizuki, Rui received a larger amount of blood from Muzan Kibutsuji than a regular Demon would which allows him to possess incredible amounts of power. This strength easily surpasses that of other Demons, since he was easily able to conquer and cohere numerous Demons of Natagumo Mountain and force them to work for him and become his family. Evidence of his strength is further shown when he was able to easily overwhelm talented fighters like Tanjiro and kill scores of Demon Slayers with little effort.
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 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.
Demonic Blood: Similar to Muzan, Rui can augment and strengthen a fellow demon's overall power and give them access to new spider-related 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 Daughter 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.
Spider Physiology: Rui, like the rest of his "family", has various spider-like traits and abilities.
- Thread Manipulation: 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.
- Steel Threads (
鋼 糸Hagane Ito?): Being the most powerful member in his "family", the sharpness and durability of his threads is powerful enough to easily break a Nichirin Blade, slice a human being into bits and pieces of mince meat and being able to easily subdue and capture fellow Demons with little trouble.
- Steel Threads (
When manipulating his threads, Rui typically simply extends the threads from his finger tips like a puppet master, however, for more complex manipulations, he uses various cats-cradle like maneuvers. He has also demonstrated the ability to increase the power of his threads.
- Cutting Thread Cage (
刻 糸 牢Kokushirō?) - Rui creates a web-shaped string around his opponent to cut him to pieces.
- Murderous Eye Basket (
殺 目 篭Ayame Kago?) - Rui creates a cage from his web around the target, that continuously shrink until it cuts the target to pieces.
- Rui's name means "accumulate" (累 rui?).
- 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 Kizuki who met the people he knew in life while disappearing; the other being Akaza.
- Rui's Blood Demon Art may have been inspired by the Imperial Arms of Lubbock from Akame ga Kill!, Cross Tail, as both grant the user the capability to wield steel threads from their fingertips, and both users use this power in a similar manner to one another.
- (To Tanjiro Kamado) "Family... A father has his role as the father, and the mother has her role as the mother, and the elder brother and sister protect their younger siblings. No matter what. At the risk of their own lives. The way I see it, if you don't understand your own role, there's no reason for you to live." 
- Kimetsu no Yaiba Manga: Vol. 7, Extra Pages.
- Kimetsu no Yaiba Manga: Chapter 51 (Page 18).
- Kimetsu no Yaiba Manga: Chapter 39 (Page 16).
- Kimetsu no Yaiba Manga: Chapter 42 (Page 10).
- Kimetsu no Yaiba Manga: Chapter 42 (Page 15).
- Cite error: Invalid
<ref>tag; no text was provided for refs named