The writers are pretty inconsistent about which swords exist in Touken Ranbu’s historical timeline for unclear reasons.
For example, the Sanjou school: After Kiwame training, Iwatooshi and Imanotsurugi were written off as being fictional swords that only exist in legend and were able to manifest in the citadel because of the Saniwa’s belief in them. However, outside of Mikazuki Munechika, who has a proven existence as a sword forged by Sanjou Munechika and currently resides in Tokyo's Nation Museum, it’s highly debatable how real the other two Sanjou swords:
In the case of Kogitsunemaru, many real swords bearing his name have been passed down through many families. However, the one described in the song 'Kokaji', which described Sanjou Munechika's fox-assigned forging of the blade, is widely thought to be a fictionalized sword, with no way to prove its authenticity due to the loss of one that could have been the real thing. However, during his Kiwame training, Kogitsunemaru travels back in time and assists Sanjou Munechika in his own forging, foxes and all, raising the question of how something clearly a legend can be considered real in the Touken Ranbu universe while something mundane, such as Yoshitsune committing suicide with Imanotsurugi, his tantou, can be dismissed as a fictionalized event. Was it because they didn’t too many fictional Sanjou swords? Why were Imanotsurugi and Iwatooshi chosen to be fictional anyway?
Ishikirimaru is another case, though it differs from Kogitsunemaru's situation. His character inspired by stories about various swords known as Ishikirimaru, the most famous of which is housed at Osaka's Ishikiri-Tsurugiya Shrine, and likely the only one that is real. Given his character, you'd expect him to mention in his Kiwame letter that he realized he's an amalgamation of multiple swords rather than a specific sword, especially given the prominence of amalgamation swords in Touken Ranbu in recent years. There's also the fact that he's described in his profile as a sword forged by Sanjou Munechika, despite the fact that every other instance of a sword named Ishikirimaru was forged by Arinari Sanjou, Munechika's son, but it's never mentioned for some reason.
With all that in mind, while it's obvious they would have chosen to implement Kogitsunemaru into the game because he's a famous sword, regardless of whether or not the sword actually exists, it leaves one to wonder why they chose two fictional swords and one amalgamation sword from the Sanjou school to implement into the game instead of ones that have a basis in history, especially if the "these swords don't actually exist" angle was going to be dropped so early on into the games story. If they needed a tantou for the Sanjou school, Ebina Kokaji, could have been used, as the sword has a documented history despite currently being lost. Takanosu Kokaji/Munechika is also a well-known work by Sanjou Munechika, being held in high regard alongside Mikazuki Munechika, and could have added another wakizashi to the games roster, which it desperately needs.
Another set of more recent examples is the Edo Sankaku, or the three great works of Edo: Suishinshi Masahide, Minamoto Kiyomaro, and Taikei Naotane. They're an unusual set of swords, said to have been forged by the smiths with whom they share a name, but, true to their names, they also inherited their smiths' personality traits, making them Touken Danshi versions of the smiths who forged them rather than Touken Danshi versions of the people who wielded them, as the Shinsengumi swords were. However, unlike the Shinsengumi swords, which did exist, none of these three swords are real swords, instead being non-specific amalgamations of swords created by three smiths. Still, like Ishikirimaru, they’re treated as real swords in the games narrative instead of amalgamations of swords, and reference real history events related to their smiths rather than the history of anyone who may have wielded them (because no one did). So which swords get to be real and for what reasons, and which not? None of these swords have been Kiwame'd yet, so there's still a chance that they'll figure out that they're not real and are just echoes of swords made by their smiths, though that seems unlikely given that, once again, the "these swords aren't real" plot line appears to have been abandoned, even though the Clap Your Hands If You Believe aspect of the Touken Danshi is still prevalent.
Imanotsurugi and Iwatooshi aside, it leaves one to wonder why the writers chose to implement a trio of amalgamations of swords instead of specific swords by these famous smiths.
Heck, Touken Ranbu even plays fast and loose on which histories are real.
Mikazuki's recollection with Honebami confirmed his status as an Ashikaga sword from the start of the game, and his recollection with Onimaru further reinforced this. All historical records, however, indicate that Mikazuki was never owned by the Ashikaga, with the exception of a legend that claims he was Ashikaga Yoshiteru's favorite sword. He was actually a sword that had been inherited and passed down through the Toyotomi and Tokugawa families for many generations; the former's ownership was verified by historical records, while the latter's was by his actual sheath design along with records. Again, his never being owned by the Ashikaga is never mentioned in his Kiwame letter, which would have been the best time to reveal it to be false if true, but it probably wouldn't matter because Onimaru seems to remember him as an Ashikaga, implying that it's the "truth" in this game's universe.
Tsurumaru Kuninaga acts like a quartet with Taikogane Sadamune, Shokudaikiri Mitsutada, and Ookurikara due to their having been passed down through the Date clan alongside him, with one adaptation even claiming that he was owned by Date Masamune himself. In reality, there is little evidence that Tsurumaru was owned at the same time as all of three, meaning that he wouldn’t have them all, and even less that he was owned by Date Masamune himself, rather than someone who happened to be a part of the Date clan. That said, this is a mistaken even fans of Touken Ranbu make, which likely influenced the decision to treat them as a group regardless.
What makes certain swords brothers, let alone family? The most likely answer is being forged by the same smith, but Sanjō school, Gō school, Kagemitsu and Nagamitsu swords, etc. share smiths but don't seem to be family, while Tarōtachi and Jirōtachi were forged by different people but are brothers. At this point, it leaves one to wonder if the Touken Danshi can just decide whether or not they want to be a family and go from there, since blood doesn't tie them together.
What factors influence the physical age of the Touken Danshi? Over the years, it has stopped being consistent due to the number of exceptions introduced in each sword-type and even some exceptions that have existed since launch, such as Kashuu Kiyomitsu and Yamato-no-Kami Yasusada looking and acting like teenagers despite the vast majority of Uchigatana resembling young adults. Even Odachi had Hotarumaru from the start, who inexplicably resembles a Tantou despite the size of his sword.
When you think about it, it's strange that most of the swords don't appear to have any special thoughts or feelings for the swordsmiths who forged them, instead focusing all of their attention on those who previously wielded them and shaping their identities around them, despite the fact that they wouldn't exist without said swordsmiths. In recent years, Touken Ranbu writers appear to have noticed this, or at least used it as an excuse to change things up writing-wise, and have created swords named after their smiths rather than those who previously wielded them, such as Suishinshi Masahide and Minamoto Kiyomaro.
A head scratcher for a lot of fans in the recent years has been how, despite having zero interactions or references to each other in the browser game, Mikazuki Munechika and Tsurumaru Kuninaga always end up in a close friendship in the adaptations, which goes beyond the obligatory "swords in the same work" interactions, especially in the musical continuity. People who are familiar with the history of the two swords will recognize that Tsurumaru Kuninaga is a descendant of the Sanjou school swords because of his swordsmith, Gojo Kuninaga, who was the grandson of Sanjou Munechika and is thought to have been Munechika's disciple, which would explain why Tsurumaru Kuninaga the sword is so strikingly similar to the Mikazuki Munechika. Both are also representative works of their respective smiths, with Tsurumaru being the most well-known for Gojo and Mikazuki for Sanjou, providing context for why they could theoretically have a relationship. However, it doesn't explain why, consistently, despite there being no basis of a relationship in the browser game itself, that all writers would have the exact same idea to pair the two swords off regardless of what role either sword has in a work, how big or small, leading fans to theorize that there's some unspoken rule from NitroPlus about the historical connection between the two having to be acknowledged in some form in an adaptation.
This has also prompted fans to wonder how, despite many years of the adaptations consistently acknowledging their historical connection to the point where it appears to be an unspoken rule on NitroPlus' part, there are no browser game interactions for two that at least acknowledge that they are familiar with each other despite other, more obscure or less logical interactions between Touken Danshi existing in the browser game.
When it comes to bridging the Sanjou and Gojo schools in interactions, no matter how many Sanjou appear in a work, Mikazuki Munechika and Tsurumaru Kuninaga are always involved with each other somehow, and Tsurumaru never develops a particularly close or meaningful relationship with any other Sanjou, if he interacts with them at all. Is it because Mikazuki is the face of his school and the representative work of Sanjou Munechika that he has a stronger bond with Tsurumaru? Is the unspoken rule applicable to Mikazuki and Tsurumaru rather than the Sanjou school in general? Is it because Tsurumaru, the sword, looks so similar to Mikazuki that he, as a Touken Danshi, can understand him better than any other Sanjou? Is it because, like the Sanjou in most cases, the Sanjou do not regard themselves as a specific group, but rather as individuals, that Tsurumaru views them in the same way, explaining his interest in Mikazuki alone? Or is it simply a writer oversight?
