I decided to put a twist on the Demon Slayer story and give the Reader a best friend who is a Demon, it'll be explained in the BIO.
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Name:
(Y/N) (L/N)

Species:
Human

Occupation:
Demon Slayer

Likes:
Cold
Snow
Cherry Blossoms
Momo
Kimimaro Kaguya
The Hashira
Quiet

Dislikes:
High heat
Sanemi's attitude
Inosuke's yelling
Zenitsu's crying
Muzan Kibutsuji
Loudness in general
Douma

Info:

You lived in a large city close to a cold region which is where your love of snow had come from.

You even made a friend with someone very unlikely.

His name was Kimimaro Kaguya, a boy who was part of the famous Kaguya family, a family of famous doctors.

The only problem you had was the fact that you could never speak.

You were born with the inability to speak which made it very hard to communicate with people.

You were often bullied because of this, people used the fact you couldn't talk against you.

You hated it, you hated the fact that you were too weak to fight back.

The only ones who ever understood you with no problem was your pet Fox Momo and of course Kimimaro.

However, your normal life came to an end when a group of Demons attacked your village.

Everyone around you was killed in cold blood and you would have died as well if it weren't for Giyuu saving you.

That teared it for you, you were so sick of being a weakling and the moment Giyu saw the determination burning in your eyes, your desire to become stronger.

Like Tanjiro he decides to let you train under his master.

The only thing you had left of your previous life was your pet fox Momo.

Momo:

Once you made it through Final Selection you were changed, you became a determined individual who would protect whatever he could.

You were also determined to find your friend Kimimaro who was not in the city when the Demons attacked.

Personality:
You come across as shy and recluse, but that is only due to your inability to speak.

You have a somewhat childish personality and enjoy almost everything in life.

It was due to your brush with death that gave you a new appreciation for life and since you have no idea when you might die you'll enjoy life however you can.

You and Rengoku are almost similar despite your opposite breathing techniques.

You love the cold and going out in the snow in your underwear does not even bother you.

You adore your little Fox Momo and animals in general.

You never give up on a goal and will try your best no matter what.

You deeply admire Tomioka and don't like it when Shinobu takes a jab at him.

Even though she's a Hashira, you'll make your displeasure known.

No one disrespects your big brother.

Powers and Abilities:
You are very fast and agile your movements have been described as Fox-like by the Hashira or other Demon Slayers.

Despite your size and build you're deceptively strong, you did slice the rock after all.

Your eyesight is far superior to other humans, you can see things from longer distances away and your hearing is unmatched.

You have such acute hearing that you can even hear muscles twitching.

Ice Breathing:
A technique of your own you derived from Water Breathing.

The forms of your breathing will be revealed in the story.

Theme Song:

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Name:
Kimimaro Kaguya

Species:
Demon

Occupation:
Upper Moon 0

Likes:
(Y/N) (L/N)
Meat Buns
Snow
Sweets
Being active

Dislikes:
Enmu
Douma
Being called weak
Not doing anything for too long

Info:
Kimimaro was born into the Kaguya Family.

A prestigious family of doctors well-known all over Japan for their amazing cures and treatments.

Unfortunately for Kimimaro he was born with a terminal disease that had no cure.

His parents often kept him confined within the house giving him treatment after treatment to try and cure him.

However, Kimimaro wanted to go out, he wanted to have a friend, if he was going to die he wanted to at least live.

So he would often sneak out of the house when his parents weren't home to go exploring.

He found you beaten and bruised and because of his own medical knowledge managed to patch you up.

He wasn't bothered by the fact you couldn't talk and he even seemed to understand you perfectly.

That is how your friendship began.

He never told you about his illness though as he did not want you to worry.

One day while he and his parents were away somewhere, he was close to death as the illness began to take it's tole.

That was when Muzan found him, he entered his room and saw how the boy would do anything to live and his indomitable spirit.

He told Kimimaro that if he became a Demon he'd live forever, an offer he Kimimaro couldn't say no to.

So the deal was struck, Muzan gave Kimimaro an extra amount of his blood and he became Upper Moon 0.

Kimimaro had no problem leaving his parents behind, they couldn't cure him anyway, plus he was a Demon now it'd be impossible for him to live a normal life.

He returned to the village you were in and was shocked to see it devestated believing his only friend was dead.

He however regained hope when he couldn't find your body and since he was a Demon he'd have an eternity to find you or at least try.

Personality:
Kimimaro as a Demon is layed back and carefree.

He rarely takes things seriously due to his power and status.

Since he rarely got out of the house he's excited to finally be able to do whatever he wants and has the power to do it.

Sure he has to eat people now but he managed to find a particular spot somewhere.

People go to this cliff and commit suicide almost regularly, enough to actually sustain him.

It's morbid, but hey as long as it works.

He is flirty with females and has gone on multiple dates with human girls, at night though, only ever at night.

Though he rarely gets serious, as Upper Moon Zero when he does it will most likely end very badly for them.

He only ever fights in self-defense or when protecting someone else.

Despite his high status he often acts friendly with the other Kizuki except Douma and Enmu cause he thinks they're creeps.

Powers and Abilities:
As Upper Moon 0 his speed strength and regeneration surpass even the other Kizuki.

He is even capable of regenerating his head if it is severed, so the only way to actually kill him is through the sun.

Blood Demon Art:
His Blood Demon Art allows him to control and manipulate his own bones into various weapons and shapes.

He can even create bones outside his own body and send them up through the ground to impale foes.

These bones are unbreakable and are as hard as steel.

He even has a full Demon form that increases his already insane physical abilities.

Full Demon Kimimaro:

Theme song:

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Here's you and Your best buddy, thanks for taking your time to read.