In the story of Demon Slayer, highly trained demon slayers fight back against the darkness with Nichirin blades and advanced breathing techniques, but often, that's not enough. The strongest demons, especially the Twelve Demon Moons, can kill Demon Slayers with impunity. Even the mighty Hashira might fall in battle.
It's clear that the Demon Slayer Corps needs another way to fight the demons, and if things had gone differently, elite shinobi could have been their ace in the hole against Muzan's monstrous legions. Unfortunately, the shinobi clans are on a decline and are usually not available to help, though Uzui Tengen and his three wives hint at what might have been.
The Decline Of The Ninja Clans In Demon Slayer
A recent Demon Slayer episode explored the world of elite shinobi in greater detail when Tengen explained his background to the Daki/Gyutaro duo, and what he said painted a grim picture of the ninjas. By now, in the relatively modern Taisho era, ninjas are a rare breed, and they often die in the line of duty, including the majority of Tengen's brothers and sisters. In fact, the ninja family's head, Tengen's father, allowed Tengen's little brother to have as many wives as desired if those wives could produce heirs. Clearly, the ninjas of Demon Slayer suffered a serious attrition rate against their enemies, and the ninjas can't keep up. Tengen is one of the few remaining ninjas, along with his three wives, and he lent Ubayashiki Kaguya his talents as loyal Hashira.
As a trained ninja, Tengen can do a number of things that even the other Hashira cannot, including advanced stealth and a notable resistance to poisons. This took Gyutaro by total surprise, and any other Hashira, even the fallen Rengoku Kyojuro, would have been at Gyutaro's mercy before long. In addition, Tengen's three beloved wives are highly skilled warriors who can take the fight to the Demon Moons in unexpected ways, including the use of large knife throwers. That, in conjunction with Tengen's sword strikes, put serious pressure on Gyutaro, and it's a shame the demon slayer corps can't fight the demon moons like that more often. Unfortunately, this is the exception, not the rule, in Demon Slayer.
What Ninjas Could Have Done Against Muzan's Demonic Hordes
Ninjas can use stealth, ambush, tracking, and unique tools to dispatch their foes, and in theory, a platoon of loyal ninjas could do a lot of good for the Demon Slayer Corps by taking on difficult jobs the demon slayers couldn't easily handle on their own. Kamado Tanjiro can track demons easily with his strong nose, but that's not typical, while ninjas could pick up the slack where demon tracking is concerned. If ninjas were regularly scouring the country for demons, then these demons could be cornered and dispatched much more quickly, and lives could be saved. The teeth-grinding demon from Season 1, for example, would not have claimed so many victims if a ninja such as Tengen's wives were tracking him. Ninja scouts could also have swept through Rui's forest and escaped alive with vital intel.
Info-gathering and tracker ninjas could have made the Demon Slayer Corps' job far easier, and even during battle, ninjas could have changed everything. Ninjas use trickery and deception to fight, and demons are used to fighting with brute force and blood arts against Nichrin blades in straightforward battles. In some fights, the demon slayer swordsman would actually just be a decoy, and the ninja would do the real work during a fight, as Tengen's fight with Daki and Gyutaro showed. Demon slayers and ninjas could have made excellent teams against demons, but alas, there are too few ninjas remaining. Under other circumstances, the demon slayer corps could have exterminated demons with impunity, and as things stand now, Tengen and his three wives will have to do. They just might be the corps' most unique, and precious, assets of all.
