Early on, the Demon Slayer anime establishes its combat system as humans versus demons. These two well-defined sides rarely blur the line that divides them. Demons are supernatural creatures with abilities like flesh regeneration, Blood Demon Arts, and overhwelming strength and speed. Accordingly, the members of the Demon Slayer Corps require special swords and breathing techniques to even these intimidating odds. The exception to this rule is Genya Shinazugawa, a Demon Slayer Corp Hashira who never learns any breathing techniques, but instead taps into his dark side to gain greater power.
Source：https://www.cbr.com/demon-slayer-genya-demon-explained/
