The following contains spoilers from the Demon Slayer: Kimetsu no Yaiba manga, by Koyoharu Gotouge, available in English through VIZ Media.
In Demon Slayer, the titular swordsmen use various Breathing Techniques to aid them in their fight against the flesh-eating Demons that plague Japan. While some Demon Slayers,like Giyu Tomioka, opt to use more common styles like Water Breathing, there are a potentially unlimited number of Breathing Techniques, thanks to Demon Slayers, like Inosuke, who have created their own style. Let’s look at how some Breathing Techniques are more unique than others.
Considering Muzan's Demons are nearly invincible, and Demon Slayers are normal humans, a tremendous boost in power is needed to eliminate even the weakest of Demons. First, a Demon Slayer must learn to use Total Concentration Breathing. This is the practice of inhaling the maximum amount of oxygen possible, in specific breathing patterns, in order to tremendously increase the user's physical and mental abilities. This ability is used in conjunction with Breathing Techniques, which are forms and styles of combat centered around the use of katana or similar handheld weapons.
Understanding Demon Slayer's Five Breathing Techniques
In the world of Demon Slayer, there are five main forms of Breathing Techniques; Water, Flame, Wind, Stone and Thunder. However, these main elements are all derived from Sun Breathing, the original Breathing Technique. The five main forms are more common because they have been more reliably passed down through the generations, whereas Sun Breathing was all but lost, save for Tanjiro's ancestors and their friendships with the first users of Sun Breathing.
Creating one's own form of Breathing Technique does not necessarily translate to superior strength or skill, considering Inosuke, who uses Beast Breathing, is nowhere near as powerful as Hashira Sanemi, who uses Wind Breathing. Interestingly, Beast Breathing itself is derived from Wind Breathing. Breathing Techniques form a sort of family tree, so to speak, where some techniques can branch off into others while being able to be traced back to the source. For example, there are multiple offshoots to Wind Breathing, as Hashira Tokito's Mist Breathing also stems from Wind.
This is not to say that someone like Giyu, the Water Hashira, is limited in his ability. Water is the form that best suits him, so he has no need to stray from it. However, users like Giyu can still make the main forms of Breathing Techniques their own by creating their own forms, as Giyu has done by creating the 11th form of Water Breathing, Dead Calm. The opposite is true for Tanjiro, in that Sun Breathing suits him much more than Water, which is why his Hinokami Kagura is more powerful than his Water Breathing forms.
Some Hashira Develop Their Own Breathing Techniques
There are some bizarre Breathing Techniques used by the Hashira, including Love, Serpent, Insect and Sound. It's no simple coincidence that Hashira Iguro has an affinity for snakes and uses Serpent Breathing. Iguro's first Breathing Technique was Water-Breathing, and his unique skillset, movements and personality resulted in his development of an offshoot Technique that is more suited to his style, Serpent Breathing. He could, in theory, teach this Breathing Technique to others and it could someday lead to Serpent Breathing being as common as its predecessor, Water Breathing.
Kanroji, the Love Hashira, uses her own unique Breathing Technique that is derived from Flame Breathing. Having initially become a Demon Slayer and Hashira to find someone to marry, it's only natural her Technique changed to a style that suits her personality and goals. In Kanroji's case, her unique Breathing Technique also necessitates a new weapon, when Demon Slayers customarily use katanas. Instead, Kanroji uses a blade that is more akin to a whip, because her movements with Love Breathing are less suited to a traditional katana.
At the end of the day, Breathing Techniques are an expression of self. Water, Flame, Wind, Thunder, and Stone are essentially the mainstream Breathing Techniques. They are no less lethal for their popularity, considering Himejima, the most powerful of all the Hashira, uses simple Stone Breathing. After all, Ubuyashiki just wants his Demon Slayers to decapitate Demons, he's not picky about what Breathing Techniques they use to do it.