I present to you, the thing that made me start thinking about all of this:

Look how pretty it is!!!! I loveeeee how the color slowly shift from a brighter gold by the hilt to a darker color at the base. I came across this image of a wakizashi (a shorter blade than a typical katana) on complete accident and when I first saw it, I gasped and was like "it's like firefly flashes whaaaaaat?!"

Then later I realized those "bursts" are flowers. It took a bit of digging, but I'm 98% sure the flowers with the small disks in the middle are chrysanthemums (kiku 菊). Chrysanthemums symbolize a lot of things (ex. beauty, nobility; the Imperial Seal of Japan is a 16 petal kiku) but what stands out the most to me is that they represent endurance, longevity, and happiness. All good things to put on a sword. Yellow chrysanthemums are also associated with the sun.

I think the other flowers with layered petals are dahlias (Tenjikubotan 天竺牡丹). The most common meaning I could find was "good taste." I think the idea is that if you give dahlias to someone, you're saying they have good taste, but I'm not totally sure. This website says that yellow dahlias are symbols of inner strength and resilience, but considering I can't find that anywhere else, I'll take that with a grain of salt. However! if that is the case, it makes perfect sense why it would be on a sheath.

I started going on a deep (deep deep) rabbit hole of tsuba (hand guards) afterward. Some are more simplistic, others are very intricate. They can depict scenes, nature, animals, plants, or family crests. All of them are so friggin' cool, oh my godddddd. I like the tsuba in Demon Slayer but having seen real-life tsuba, I feel like the potential for symbolism and overall cool factor was a bit lost. It makes sense why the overall designs were more simple (easier to distinguish, not having to worry about tiny details, etc) but the possibilities and creativity is endless! There's a whole wiki commons page with tsuba, which led me to the MET website which has even MORE. I browsed through, uh, pretty much all 1000+ of them, and here are a very highlights! (most of these are the less traditional ones tho)

Some of these I picked out because they look similar/have similar motifs to tsuba in canon, some just because they're neat, and some because the craftsmanship is insane???? The one ginko leaves and dew with the tiny lines and the way it folds???? the masks???? The dark blue one with tiny gold stars and snowflakes ingraved into it??? Almost all of these are metal, but the lily pad is leather and lacquer which is just so cool to me. My favorite is the bamboo and plum flower one, it's simpler than most of them, but it's got a quiet and almost bold feel to it.

I had wayyyyy more, but if I included every single tsuba I liked, I would have no room for other images, lol.

So when I was looking into firefly symbolism, one thing I read was that they were used on sword furnishing/decorations. At the time, I couldn't find an example (not for lack of trying), but I was finally able to find a few. There were a LOT of tsuba with insects on them and some of them could have been fireflies, but because of stylistic choices, I couldn't confirm it.

The one on the left was the first I found. It has fireflies, butterflies, AND a dragonfly. Creators often inscribe their name somewhere on the tsuba and the reverse side says “Spring by Mitsutada, summer by Ichijō, fall by Tōjō." Butterflies are a symbol of spring, fireflies summer, and dragonflies autumn. Love it when artists are artistic lol

(Dragonflies and butterflies are very common motifs on tsuba. An old name for dragonflies is “victory insect” [katsumushi] because they are quick in killing their enemies and became a popular symbol among samurai. They are still a very common motif on modern day kendo tsuba. Butterflies are associated with a lot of things [grace, love, fleetingness of beauty in life] but notably, transformation/metamorphosis and in Shintoism, they were a symbol of the soul's journey into the afterlife or embodied the souls of the deceased. All of which puts Shinobu in an ENTIRELY new light.)

The tsuba in the middle has two fireflies on the edge, each on the top and bottom (it took me a while to see them because it's so dark in color), and the gold lines are grasses. The one on the right also depicts fireflies in grasses, but the bugs are much easier to spot. I really like this one actually, it's not as flashy (haha) but like with the bamboo/plum tsuba, I like the simple but quietly bold approach.

I should probably move on from tsuba since they are far from the only pretty part of a sword. There are many parts to a katana and even looking at a diagram, I still don't even know what have of them are.

For example, menuki (not to be confused with the mekugi, the peg that holds the handle to the base of the actual blade) are ornaments on the handle. Originally, they were meant to cover the mekugi, but over time, both became visible. The swords in Demon Slayer (and most if not all manga/anime) don't have them, probably again to keep them simpler. You could say the canon reason is that the swordsmiths don't have the time to put as much detail on swords.

While they're cool, I'm not as enamored with them. Possibly because I can't help but wonder if a katana with menuki would be uncomfortable to hold and/or if you get blisters from them. though many of them are rather flat and partially covered by the cords, so maybe it's not that much of a problem. Regardless, they are very neat

The bat one is really cool to me, I like the way it stands out. If you can't tell what's on the other sword, it's a snake. I'm pretty sure the ones in the middle are bamboo leaves.

If the trio were to have menuki, I would definitely use it to add a little extra subtle layer of symbolism. It's not entirely clear how much the swordsmiths know about the slayer they are assigned to, but I figure that would utilize the little information they have in order to personalize a sword. Meaning they might use people's names to add some subtle details. So...

Hanako's name means "flower" (hana 花) and "child" (ko 子, a common ending to a girl's name). There's probably other flowers with symbolism that more closely fits her character, but I think the most fitting one to pick would be a peach blossom.

Takeo's name means "bamboo" (take 竹) "man" (o 雄, I've seen this also translated as "manly" or "heroic"). I think that the bamboo leaf menuki above would fit very nicely.

Senjuro is a bit less obvious. His name means "thousand" (sen 千) "longevity" (ju 寿) "son" (ro 郎). There are a lot things that represent longevity, and originally I was thinking about a chrysanthemum, but I looked a little bit more into cranes which are another common symbol. Cranes are said to live a thousand years and it's become a tradition to fold origami cranes to wish an ill person recovery, specifically one thousand cranes within a year. So I think cranes are a better fit. (oh god now I'm picturing little Senjuro and Kyojuro folding origami cranes for their mother and stringing the garlands around her room ;-;)

Um. Anyway. Back to swords.

here are a couple swords in their sheaths:

The ones on the left are both katana, the blade is from the katana on the top left, and on the top right is another wakizashi and the bottom right is a tanto (a dagger). The katana on the top left is very unusual because it's disguised as a walking stick and oh my goddddd that cherry bark is sooooo pretty, it's the prettiest kind of bark I don't take criticism. But aside from that, its hamon (the lighter pattern on the blade) really stand out to me, sort of flowy and bubbly. I'll be the first to admit I don't fully understand hamon, but this article does a good overview and has some nice close-ups.

The brown katana doesn't stick out much at first, but I love the patterns on the sheath, which is a lacquer technique called seikaiha-nuri (literally “blue waves lacquer”). It also has lovely little detailing of insects and flowers on the fittings that I didn't notice at first.

The wakizashi and tanto are both really pretty. The maple leaves with varying colors is so cool and the wakizashi has so many cool insects on it. The tanto has beautiful flower detailing and some of them are a family crest (kamon) of the Toyotomi Family. Family crests are really common symbols on swords obviously, but you can find them elsewhere even today in architecture, tenugui (handtowels), etc. They're both lovely, but I find myself leaning toward the simpler, subtler design of the brown katana.

If you're at all curious about the choices behind the trio's swords, the brown cords and yellow same (what's under the cords) are mean to evoke a firefly's glow behind its shell. Hanako's brown sheath is also meant to be the "shell" for her yellow sword. Sengoku era Rengoku, Shinjuro, and Kyojuro all have the same sheath pattern so that's why Senjuro also has it, it's traditional. I was a bit stumped for Takeo but then I realized that fireflies are associated with grassy banks, so I went with dark green (also look, it matches his scarf and Tanjiro's haori now). The matching tsubas with three fireflies is self explanatory.

Can you tell I have overthought this a lot?