u/Calm_Protection_2641

Think about it: in the world of Demon Slayer, set in Taishō-era Japan, wouldn’t a girl without a male companion (since Tanjiro couldn’t appear due to being an Oni and restricted by its biological and social limitations) be seen as suspicious and have a target placed on her back? Moreover, within the Demon Slayer Corps, it’s easier to see men than women, and this becomes even more evident during the Hashira training arc. The world of fighting Oni depends on strength, skill, and technique, so Nezuko, as a woman, would have received less encouragement to enter or remain on the front lines of combat. Any scratch Nezuko received would draw more concern than any injury Tanjiro would, both in this AU and in the canonical universe, because she is a woman and thus primarily associated with delicacy and domestic life rather than the demands of strength.

Even though she and Tanjiro canonically share the same personality traits—kindness, empathy, attachment to family and normal life, altruism, and strong morality—social expectations are different for her. She is the second child, and therefore not the heir to the Hinokami Kagura, and also a woman, who is generally considered a burden in the most intense battlefields. This stems from the historical pattern of men going to war while women are preserved to bear, care for, and protect children, as well as from sexual dimorphism. Exceptions like Shinobu are more due to compensatory intelligence adapted to biological limitations in strength, and physical exceptions like Mitsuri are even rarer, being genetic anomalies. Most members of the Demon Slayer Corps are male, and even among the upper ranks of demons this pattern appears. Exceptions like Nakime, who is not a fighter and remains quite passive (though her control over the Infinity Castle is overpowered), and Daki, who isn’t strong on her own but because of her brother—the true Upper Rank Six, who challenged the Hashira assigned to defeat him—are overshadowed in power and impact, with leadership shifting once he appears. Both exceptions are not dominant presences on their own.

Nezuko would inevitably be underestimated, for better and for worse, and having an older brother who is an Oni would only make things more stigmatizing. She would likely receive more protection from those around her than Tanjiro did in canon, but in doing so, people would become overprotective and hinder her ability to excel in the intense battles she would inevitably face. Some warnings would be valid due to real observed limits, but others would stem from underestimation. For these reasons, Nezuko would train harder to prove she can stand equal to other Demon Slayers. Additionally, those who try too hard to protect her would be putting themselves in danger and could die—something she would want to avoid out of kindness and her desire to save Tanjiro.

Speaking of Tanjiro, he likely would have received more of Muzan’s blood in an attempted killing due to his resemblance to Yoriichi, which might make him more powerful than demon Nezuko—or perhaps not. We don’t know their true potential under equal conditions because the circumstances were not the same. Canon Nezuko was limited by the tools she had: she wasn’t taught to use a sword, had to fight hand-to-hand, restrained herself to preserve her humanity, and even abstained from consuming humans and demons. Despite this, she still contributed to the defeat of Upper Ranks Six and Four and adapted constantly. Her Blood Demon Art is tied to fire, and the Kamado family has performed the Hinokami Kagura for centuries, meaning Nezuko has a genetic predisposition to become a Sun Breathing user, just as the Rengoku family is tied to Flame Breathing. In contrast, Tanjiro had to receive all of Muzan’s blood to surpass demon Nezuko. Therefore, the parameters are not equal.

Regardless of whether demon Tanjiro is stronger than demon Nezuko, he would naturally be stronger than human Nezuko. And Nezuko would still have to fulfill the roles of younger sister, protector, and Demon Slayer. Tanjiro would be a greater target for Muzan precisely because of his resemblance to Yoriichi and the hanafuda earrings. Because of this, Nezuko would have to exercise extreme caution and endure harsher situations, such as being pursued more frequently by Oni and facing increasingly powerful ones sent by Muzan. The risk to lives would be greater, putting pressure on Nezuko to grow stronger to prevent tragedies—something that would not always succeed.

There is a theory that Nezuko inherited her father’s talent, given her feats as a demon and Tanjiro’s own acknowledgment of his inferiority compared to his father and Yoriichi in performing the Hinokami Kagura. Nezuko is Tanjuro’s second child, just as Yoriichi was the second son of Mr. Tsugikuni and Akeno. Furthermore, if the goal were to hide someone’s talent to avoid a tragedy like Yoriichi’s, making her a woman and not giving her the red eyes and hair—the hallmark of Sun Breathing and a symbol of good fortune according to Haganezuka—would be an effective disguise.

Nezuko being underestimated would become her greatest weapon, whether she realizes it or not.

Tanjiro, in the Entertainment District arc, used a mix of Water + Sun Breathing, but only once, for a few seconds, in a raw and unrefined way—not as a new Breathing style derived from Water + Sun, but rather as two separate styles used together at the same time.

Would these four reach Hashira level (Hashira level = Yoriichi-like similarity) just through training, or would their still-developing bodies be overwhelmed and unable to keep up with the intensity of the training? Would the training be harsher than Urokodaki’s, or would Kokushibō’s centuries of experience make it more controlled, efficient, and high-quality instead? Kokushibō knows the Transparent World, Total Concentration Constant Breathing, uses Moon Breathing, has knowledge of Sun Breathing (except possibly the 13th form created by Yoriichi during his battle against Muzan), is familiar with Mist Breathing as shown in his canonical battle against Muichiro, and likely possesses other knowledge from his experience. No demon has ever trained humans before.

Would the vast difference in strength between the mentor and the still-developing bodies of the four cause their innate genetic potential to be awakened through constant training stimuli (making their base form overpowered since they are being trained from before puberty), potentially pushing them close to Yoriichi’s level? Or is the human body too fragile to adapt under the guidance of an interspecies master far beyond human limits?

The only things Kokushibō does not know are the Red Blade and the Selfless State.

Would the four develop the Demon Slayer Mark, or given Kokushibō’s personal resentment toward the shortened lifespan it causes, would he intentionally train them to become strong enough without needing the Mark at first, taking advantage of their childhood to shape their bodies so their baseline strength is already optimized?

Nezuko can exist both as a demon and a human.