A sudden spark of inspiration hit me for this one and it basically wrote itself. I hope you enjoy this short idea of mine!
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Thump. Thump. Thump.
Three times in a row before a few seconds passed by in silence until the rhythm started anew. It was the sound of life itself that all beings in this world shared with each other. From the birds in the skies to the fishes in the sea. All creatures possessed a beating heart within their chests without exception.
For as long as Zenitsu could remember he has been able to listen to the rhythm of life, long before his grandpa took him in and granted thunder to him. The breathing technique he was taught by the old Hashira worked well together with his advanced hearing, increasing the tunes the golden boy could pick up from the world around him.
Alongside beating hearts, he was now able to hear the contradictions of a living being's muscles, making it possible for him to predict their every move long before they took action. On rare occasions, he was even able to listen to a person's thoughts. A wonderful idea many would probably think and to some extent it even was, yet this ability also caused a paradox he was unable to wrap his head around. All creatures in this world had a heart, but there was one being that was different from the rest. Demons.
Thump. Thump.
Instead of three times how it was supposed to be, a demon's heart only drummed twice before silence fell upon it, not only for mere seconds but minutes. Zenitsu still remembered the time he encountered his first demon, how frightened he was back then, unable to move a single inch as the sound of death creped into his ears.
Thump. Thump.
It was his grandfather who saved his life on that day with a swift cut through the demon's neck despite the leg the old man was missing. On top of that, the former Hashira took even care of Zenitsu's money problems back then and afterwards offered him a place to stay. Needless to say that the golden boy accepted without hesitation, for he had never experienced such kindness before.
However, a few years later, Zenitsu encountered a person who could easily rival his old man in terms of kindness, maybe even surpass him. It was the same person though that caused so much confusion within his heart.
Even when they had barely known each other for less than an hour, Zenitsu could already tell that Tanjiro was without a doubt a nice and caring guy. There was a slight divergence whenever the crimson-haired teen's heart followed the rhythm Zenitsu had become so accustomed to. It was stronger and more vibrant, captivating in a way the golden boy had no clue how to explain. Not to himself nor to anyone else. Tanjiro was simply the nicest guy Zenitsu had ever met, not even losing his smile when one of his ribs was broken.
Still, from Tanjiro's back, from within the wooden box he was carrying around, Zenitsu heard the sound of death only demons could create. At first, he thought that his new friend wasn't aware of the fact that he was journeying with a demon, but the thunder breath user dismissed that thought faster than a bolt of lightning could travel across the sky. Tanjiro had to know what was inside the box, for there was no way he wasn't aware of his companion's condition. Zenitsu could only take wild guesses at best unless he would openly ask. It was not his place to ask Tanjiro something this private, at least that was the impression the golden boy had.
So he would wait and observe until the time was right, listening as the sounds of life and death intertwined, creating a new rhythm Zenitsu had never heard before. The one thing he knew though was that the demon on Tanjiro's back was different from those he had met before, not because of her one-of-a-kind travelling methods, but because the emotions Zenitsu could hear from her were filled with nothing but Kindness.