Tanjiro Kamado and Nezuko Kamado vs Rui was a battle that took place on Natagumo Mountain. It focuses on the battle between Demon Slayer Tanjiro Kamado, and Demon Nezuko Kamado against Lower Rank 5 Rui.
Tanjiro tells Rui that he doesn't have a real family bond, only a fake one made out of fear, mistrust and violence, and not a bond made of love, kindness and compassion. This enrages Rui who then attempts to cut Tanjiro to pieces with his threads. Tanjiro dodges the steel-like threads to his best efforts. Rui then surrounds Tanjiro with a barrage of threads. Tanjiro closes his eyes, expecting to be cut up, but he was unharmed.
Both Tanjiro and Rui are surprised, as its revealed that Nezuko was the one who protected Tanjiro from the threads, while letting her body receive the full cutting force of the threads. Tanjiro screams Nezuko's name in horror, and then he quickly carries her to safety. Rui realizes that the two are siblings. He is shocked that even though Nezuko was a demon, she still protected Tanjiro, a human. Rui realizes that their bond was genuine. He declares that he wants that bond, his eyes shining with envy.
The Spider Demon Daughter, who was listening to this, begs Rui not to abandon her, as she was his older sister. In response, Rui cuts off the daughter's head, telling her to shut up. He then says coldly that she and everyone else in the "family" never fulfilled their roles, not even once. He then tells the daughter if she wanted to prove her worth to him, she should go and kill all of the Demon Slayers on Natagumo Mountain. The daughter obeys, then picks up her head, and leaves the scene.
Rui then attempts to negotiate with Tanjiro, saying that he was really moved by Tanjiro and Nezuko's bond. Rui then says that he will let Tanjiro leave alive, but only if he lets him take Nezuko. Tanjiro immediately says no, saying that Nezuko is a living being with feelings, emotions, and a heart. Rui then says it would be alright, saying that he will forge a bond of fear between him and Nezuko. Tanjiro declares he will never hand over Nezuko, and that he will sever Rui's head. Rui compliments Tanjiro on his spirit and courage. He reveals to Tanjiro that he is actually Lower Rank 5 of the Twelve Kizuki. Rui then says he would like to see Tanjiro try to defeat him in battle.
Rui uses his threads to pull Nezuko over to her side, saying now that he had his sister, Tanjiro's role was to die. Tanjiro then charges straight at Rui with his broken Nichirin Blade. Rui uses his threads to attack Tanjiro while Nezuko attempts to escape from Rui's grasp, even clawing at his face. As Tanjiro dodges Rui's attack, he notices that Rui isn't holding Nezuko anymore. Just then, a spurt of blood falls on Tanjiro's blade. He looks up and sees Nezuko strung up midair by Rui's threads, with the threads digging into her skin, badly wounding her. Rui says that the threads won't kill Nezuko, since she's a demon, but she'll have to be taught a lesson. He continues to say that if Nezuko doesn't behave, he will leave her like that until sunrise to be burned and disintegrated. Tanjiro yells at Rui, demanding that he let Nezuko go. Rui then uses his threads to trip Tanjiro and make him collapse to the ground. He then kicks Tanjiro in the stomach which sends him flying to a bush.
Rui then beats up Tanjiro, and this continues until Tanjiro gets a chance to cut Rui's head off. But his blade doesn't even scratch Rui's neck. Rui declares that Tanjiro has no chance of defeating him, and he then gives another kick to Tanjiro, making him crash into a nearby tree. Nezuko who was witnessing this struggles in her strung-up state, screams and attempts to break free from the threads. Rui says she's making too much noise, and then he tells her to quiet down. The threads tighten and cut deeper into Nezuko's flesh, causing her to scream in pain as she becomes even more injured. Tanjiro then telepathically, tells Nezuko not to get emotional, or she would be playing right into Rui's hands. Nezuko then seemingly falls into a sleep-like state. Rui notes that Nezuko has a unique aura and is different from other demons. Tanjiro tells himself to calm down and to control his breathing. He realizes that in order to defeat Rui, he must use the final form of the Water Breathing. Tanjiro uses the Total Concentration: Water Breathing, Tenth Form: Constant Flux. Rui thinks to himself that if this continued, he would be in trouble. Rui then pumps his own blood into his threads, turning them red and increasing the power to his absolute limit. Rui uses his Blood Demon Art, Cutting Thread Cage to Tanjiro which completely surrounds him. Tanjiro can tell by the scent of the threads that he cannot cut them at his current strength.
Thinking that he might die from the threads, Tanjiro's life flashes before his eyes. He then remembers the Hinokami Kagura his father, Tanjuro Kamado would perform at New Year's Eve every year. He also remembers what his father said about making sure that the hanafuda earrings and the Hinokami Kagura would get passed down to Tanjiro uninterrupted. Tanjiro remembers the promise he made to his father. Then, Tanjiro switches from the Water Breathing to the Hinokami Kagura, Dance, the dragon of water gracefully turning into a burst of fire and sun. The Breathing Style that had been passed down the Kamado family for generations, was powerful enough to cut Rui's strongest threads. Rui started to panic after seeing this. He started to back away from Tanjiro while he counterattacked with more threads. The clash between threads and Hinokami continued until Tanjiro used his enhanced sense of smell to find the opening thread. Tanjiro knows, that even if he is struck down by Rui's threads after the final strike, he must defeat Rui no matter what.
Meanwhile in Nezuko's consciousness, Kie Kamado appears and reaches out to her pleading that she helps her brother, or else he would die. Nezuko then wakes up, and she unlocks the ability to use Blood Demon Art, Exploding Blood. Nezuko turns the blood on Rui's threads that were entangling her into pink and crimson red flames that incinerated the threads that were about to strike Tanjiro, while also leaving a few drops of Nezuko's blood on Tanjiro's Nichirin Blade. Rui is surprised that the threads were burned by the pink flames. Tanjiro then gets his blade's range into the opening thread. Tanjiro then exclaims that the bond between him and Nezuko can never be severed by anyone.
As he exclaimed this, the drops of Nezuko's blood on the broken blade turned into fire and then the heat of the fire is absorbed by the blade, allowing the color of the blade to briefly turn crimson red from black. Tanjiro then severs Rui's head from his body with his blade, which had combined the powers of Tanjiro and Nezuko as one, like how the sibling's hearts were as one.
After the battle, Nezuko is released from Rui's entanglement. She collapses to the ground, exhausted. Tanjiro also collapses to the ground due to forcibly switching between the Water Breathing and the Hinokami Kagura. As he crawls towards Nezuko, he realizes that Rui wasn't giving off the ash-like smell when demons disintegrate. It is then revealed that Rui quickly cut his own head with his threads before Tanjiro could. Rui reattaches his head, saying that he is annoyed at how Nezuko's Blood Demon Art damaged him but not Tanjiro. Rui has now become even more angry, and he attempts to finish off Tanjiro with Blood Demon Art, Murderous Eye Basket. Just as Tanjiro was about to be cut into pieces, he is saved by the Water Hashira Giyu Tomioka. Giyu compliments Tanjiro for holding his own against a Twelve Kizuki for this long.
He then tells Tanjiro to rest up and protect his sister while he deals with the rest. Rui, infuriated that more Demon Slayers had shown up, prepares his most powerful attack: Blood Demon Art, Cutting Thread Rotation. Giyu uses the Total Concentration: Water Breathing, Eleventh Form: Dead Calm to block it, nullifying the attack. Rui is shocked that Giyu nullified his most powerful attack, without even trying. He tries to attack Giyu with more threads, but before he could do so, Giyu had cut Rui's head in one simple stroke. Rui was shocked by this, not wanting to believe what just happened. Rui then disintegrates into ash as he tried to reach Tanjiro and Nezuko, with Tanjiro resting his hand onto Rui's robe.