Kokushibō and Tanjirō, Nezuko, Yuichirō & Muichirō (fanfic prompt or analize)
I believe it could work, but it would be highly circumstantial in order to encompass both the Kamado siblings and the Tokito siblings. To maintain all four, the bond with the Kamado siblings would have to arise before the bond with the Tokito siblings, and since Yuichiro was killed at around eleven/ten years old, Kokushibo’s bond with the Kamado family would need to happen before Nezuko reached eleven/ten years of age.
First, to address the elephant in the room, there is the issue of Muzan’s control, which is not absolute, although it is very strong, and the question of whether Muzan would instantly kill Kokushibo.
Muzan would try to kill Kokushibo, but in this context of Kokushibo forming bonds with the Kamado family, his body would already be in the process of undoing the curse, which would allow him to barely survive. Muzan does not constantly monitor his demons—he is not omnipotent—which creates a window for Kokushibo to build bonds with the Kamado family during the periods when Muzan is not observing him. In addition, he left Kokushibo alone for several decades after Yoriichi’s death, so it is not impossible for a similar period of peace to occur again, especially since Kokushibo is essentially a business partner to him. Despite the immense control he has, it is not absolute. Nezuko, Tamayo, and Yushiro are not under Muzan’s control.
I asked why exactly Nezuko is not under Muzan’s control, and one of the most interesting answers I found was from user u/Inevitable-Freedom-9:
>First of all, Muzan is not omnipotent. The Fanbook states that he can only read the minds of demons when they are within his field of view. The only times we see him gain information about his demons are when they die, or when they are right next to him. The fanbook also states that his mental and physical influence over other demons decreases significantly with distance, and he has literally not been anywhere near Nezuko since he turned her, except for the one time he met Tanjiro, when he did not know Nezuko was there.
>Second, Kibutsuji’s Death Curse is activated when Muzan’s name is spoken. Nezuko never did that.
>Third, Tamayo helped undo the curse.
>Fourth, overcoming the sun also helped break the curse, since her biology transformed into something separate from Muzan himself.
This is interesting—especially the first points. I am not sure if it is true, since I do not have the fanbook, but it is there for verification.
Even without the fanbook, the anime and manga provide some hints:
During the Kokushibo fight, Muzan has to mentally ask Kokushibo what is going on and how many Hashira he has killed. Muzan cannot read his mind and automatically find out.
We never see Muzan interact mentally with a demon if they are not right next to him. He does not reprimand Akaza for failing to kill Tanjiro until Akaza meets him to report it. When he executes the Lower Moons, he has them all in the same room directly in front of him. He does not already know the information Gyokko had uncovered and was about to tell him.
On multiple occasions, he is shown to only gain awareness of his demons when they die (Akaza appearing to kill Tanjiro only after Enmu’s death, knowing how Gyutaro and Hantengu died but not knowing they were fighting beforehand and not interfering, appearing to mentally belittle Akaza after he was beheaded and actively dying, etc.).
Additionally, Muzan’s cells only activate automatically when a demon says his name or tries to reveal information about him—not due to thoughts of betrayal (an ideological misalignment). Another loophole.
The canon leaves gaps in Muzan’s control over demons, but in every case where the curse is broken, it is emphasized that the circumstances are highly specific.
Once the curse is broken, Muzan no longer has control over the demon.
With Kokushibo, I cannot imagine him breaking the curse in a clean way, considering his condemnable past and how much that past challenges the sustainability of bonds. The parallel I imagine is him choosing to attach himself to the rejected Sun Breathing family (the Kamado family) and suffering physical punishment from a higher authority figure (Muzan) and surviving, just as he suffered physical punishment (such as slaps) in childhood from his father (a higher authority figure) for attaching himself to the rejected Sun Breathing family (his brother, Yoriichi), and surviving.
Secondly, there is the question of why Kokushibo would attempt to form a bond with the Kamado family, and later add the Tokito siblings. Someone would have to break Kokushibo’s emotional barrier for this to happen. Given the current canon circumstances, this is impossible. Kokushibo is too stubborn to change his mind. His world is strength and combat. For his desire for family to be restored, there would need to be a slow emotional development with someone strong enough to rival him and capable of defeating him if he tries to kill them, but also someone willing to give him a chance and let him experience what family feels like without rubbing it in his face or directly opposing Muzan Kibutsuji (which would otherwise cause Kokushibo to keep his guard up), meaning this person must be calm and peaceful. This person must be a refuge for Kokushibo, even if he does not admit it, while Muzan is not monitoring him, and during this time his repressed love for Yoriichi would find an opening.
The only person capable of this is Tanjuro Kamado, Tanjiro and Nezuko’s father. Tanjuro is 1) strong enough to rival Kokushibo and restrain him, and the younger he is, the less affected he is by illness, giving him the opportunity to show his true potential, 2) has no intention of killing, 3) is kind, gentle, and lives a simple life, 4) resembles Yoriichi as a “second chance,” 5) has a family consisting of a wife and six children, into which he could allow Kokushibo to be integrated—but only after Kokushibo had already become familiar with them over years of visits and after it became clear that the demon had given up trying to kill them, with a silent agreement along the lines of “you don’t kill me and don’t use a Nichirin blade, I don’t kill you or your family” forming an unspoken truce. Tanjuro may be calm and peaceful, but he would certainly defend his family if Kokushibo tried to harm them.
The transition from strength to familial coexistence can occur here. Kokushibo is a killer, but he is also polite, patient, has honor in sword duels and battles, and follows a distorted code of honor. In these circumstances, meeting a young Tanjuro allows him to witness the boy’s growth and strength. We do not know Tanjuro’s true strength, so there is room for creative freedom; however, Tanjiro mentioned that his own Hinokami Kagura does not compare to his father’s, and in the Entertainment District arc he stated that his father was born with the Demon Slayer Mark, suggesting that Tanjuro was a powerful individual, whether affected by illness or not. A young Tanjuro defeating Kokushibo would parallel child Yoriichi defeating Michikatsu’s instructor. This alone would already be a reality-shifting shock. But since Tanjuro has no intention to kill, Kokushibo has space to process the situation and gradually become less hostile as he returns to Mount Kumotori to visit and duel him repeatedly. A young Tanjuro also gives Kokushibo time to observe his growth and feel important, which is essential for him to avoid hostility toward Tanjiro, and provides him with a model of affection and fatherhood to emulate when adopting the Kamado siblings after Muzan’s massacre.
And speaking of the massacre itself, it is possible it would not be the same, because if Muzan discovers what Kokushibo has been doing with that family…
***
Substantively speaking, wouldn’t the Kamado family massacre be… darker with this prior bond established between Kokushibo and the family, which later evolved into the adoption of the survivors? In canon, Muzan went to a random family, killed everyone, and left one girl alive to be turned into a demon in order to conquer the sun and devour corpses—and by coincidence, that family were Sun Breathing users. Here, the scenario is less likely to remain the same, since the highest-ranking member of the Twelve Kizuki and his business partner—someone Muzan trusts—had developed ties with this family, which originally began as nothing more than a rumor of Sun Breathing users that Kokushibo followed years earlier to investigate.
What would be more likely to happen? And which option gives more weight, meaning, and substance to these bonds while respecting the canon worldbuilding? (Something that makes you feel like the pieces fit together and reconciles canon with this alternate path.) A cold, technical, analytical, rational criterion can be used.
Muzan does not find out, the massacre proceeds as in canon, and Kokushibo secretly comes to adopt Tanjiro and Nezuko while Muzan is not monitoring his mind, hoping no Demon meetings occur while he searches for Tamayo to break the curse.
Muzan finds out, kills Kokushibo, but since Kokushibo had unknowingly ingested the Blue Spider Lily with the Kamado family—who had access to this plant—he is able to return to life, breaking Muzan’s curse and going back to Mount Kumotori, where Muzan went personally to kill the family. Tanjuro defends the survivors and later receives help from Kokushibo when he arrives; Muzan escapes during their fight, and Tanjuro dies from exhaustion of power.
Muzan finds out, tries to “teach Kokushibo a lesson” by beating him, then drags him by the hair with his injured and broken body to Mount Kumotori and forces him to watch helplessly as he brutally massacres the family before his eyes. Tanjuro—who, as someone Kokushibo initially met with the intention to kill for being a Sun Breather but ultimately spared or could not kill, and who found a sense of equality in him through a slow, calm development shaped by rural life (an equality he never found in Yoriichi and only acknowledged in death)—had trained under him over the years and thus become stronger. He arrives because he was outside the house at the time (perhaps distracted by the Twelve Kizuki attacking the village under Muzan’s orders) and manages to hold Muzan back and even injure him, allowing Kokushibo to break the curse as an analogy to Tamayo’s situation. They then fight, and to prevent the situation from worsening—already emotionally unbearable—it becomes necessary to fake the deaths of the survivors, and Tanjuro ultimately dies from exhaustion of power.
Would Kokushibo breaking the curse realistically be more plausible?
***
When the Kamado siblings survive the massacre two years earlier than in canon (Tanjiro and Nezuko being 13 and 12 in canon, but 11 and 10 in this alternate timeline), Kokushibo would adopt them, and later the three of them would meet the Tokito twins, also orphaned. Since Kokushibo is an adult and the twins are in the same age range as Tanjiro and Nezuko, adoption would be natural. Another brief period of peace begins, and from there the prologue of the story ends.
Tanjiro and Nezuko are linked to Kokushibo’s brother Yoriichi, while Yuichiro and Muichiro are linked to Kokushibo himself. They are pairs of siblings. Both Kokushibo and his brother, as well as his own children, were pairs of two. Tanjiro and Nezuko fit the adoptive-family aspect of Yoriichi and Kokushibo, while Yuichiro and Muichiro fit the biological-family aspect of Kokushibo’s lineage. Both pairs of siblings are orphans who grew up with their biological parents until a certain point in childhood, maintaining the shonen trope of the orphan protagonist maturing early. Tanjiro and Nezuko may unconsciously parallel Michikatsu’s children (a boy and an infant of indeterminate sex), while Yuichiro and Muichiro more clearly parallel Yoriichi and Michikatsu as twins.
Demon Slayer contains many parallels and strong symbolic symmetry: Yoriichi and Kokushibo, Zenitsu and Kaigaku, Mitsuri (love) and Zohakuten (hatred), Tanjiro and Nezuko and Gyutaro and Daki, Tanjiro and Nezuko and Sanemi and Genya, etc.
The Kamado siblings and the Tokito siblings as Kokushibo’s redemption and ascension fit the typical symmetry of Demon Slayer’s parallels.
***
Just because he has lived for centuries doesn’t mean he is experienced in everything in life. He is experienced in combat, but a novice when it comes to emotions, family bonds, and being a teacher (he has no apprentice since he focuses on himself).
Furthermore, it is not about someone trying to “save him.” It is a circumstance where he can have a second chance without the weight of his past overwhelming him, and this includes the absence of Demon Slayers. It is a situation where the environment is calm—a safe refuge—where he takes his time and then chooses this second chance. It is more like a hand being extended, not someone going out to try to save him; it is Kokushibo going toward it himself.
In addition, throughout all these years, no one has reached Yoriichi’s level in order to confront him. Tanjuro is an outlier variable in Kokushibo’s life. He is capable of confronting Kokushibo physically, but he also has a calm and just personality. Kokushibo’s story with Tanjuro is not written in stone, and the younger he is, the more evident this becomes.
Another caveat concerns the very meaning of redemption and ascension. Redemption does not mean being forgiven by one’s victims or avoiding the consequences of one’s actions; it means no longer being the same bad person who committed those wrongs. Just because he would begin to care for and try to be better toward Tanjiro, Nezuko, Yuichiro, and Muichiro does not mean he would lose his murderous, cruel, and distorted tendencies, nor that others would be wrong to distrust him. The character could achieve an internal redemption, but not an external one.
Furthermore, even if the character were to fundamentally change in personality due to these altered events, no one would feel satisfied accepting him, since he would still be a danger based on his concrete past. It would only be unacceptable if the characters shifted their blame and hatred onto the children—who are more vulnerable—in order to hurt him emotionally. He would be aware of this and, knowing the social stigma that would arise if his relationship with the Kamado and Tokito families became known, he would try to conceal the children’s involvement with him, even while caring for them.
The character knows he will go to hell regardless when he dies, so why not make use of these bonds and give meaning to his own death?