30 year old half demon, 5 foot 9 inches tall with slightly tan skin, pure green eyes, pure red hair. Uses Death Breathing, fully custom 10 Form fighting style that somewhat relies on his Blood Demon Art. Also uses versatile Blood Demon Art against Demons, as well as a blood magic enchanted scythe (Dante's Inferno death's scythe), dubbed "Blood Reaper" that can also shift into a naginata or sword at his will, and a pair of similarly enchanted kunai, "Onyx" and "Pearl", that will return to his hand when he reaches out for them.
The OC is actually a fluke "Half Demon" created in the same way that Blade was as a Daywalker. Needless to say, I feel the same way about Demons that Blade does Vampires, but I (my character) DOES acknowledge the fact that not all Demons are Evil, such as Tamayo and Yushiro. After the birth, my Mother took me and carefully made her way back home, explaining the event to her husband, who convinced her to remain with him, using his and his family's influence to get her new situation accepted in the small village we lived in. The other villagers actually FREELY OFFERED to donate their blood to feed her and I, only to find out that just blood wasn't enough; she still needed ACTUAL food (think Twilight hybrids)! As the years passed, it was also discovered that she could tolerate low levels of sunlight, whereas I could survive like an actual human.
One day while I was merely 14, Muzan Kibutsuji, along with the Upper One and Upper Two Kizuki, tracked my mother down and proceeded to coldly slaughter my family while the two Kizuki held my mother and I immobilized and forcing us to watch. That's when my Demon Form first manifested... after the pain had quickly subsided,  I managed to severely injure not only Upper Two, but also Upper One AND KIBUTSUJI HIMSELF, forcing the trio to actually retreat. Along with my first Demon Form transformation, a scythe (Blood Reaper) had manifested, which I had used with unnatural ease and precision.
After cleaning up the aftermath, with help from the villagers, I was actually advised by the village elder to apply for the Demon Slayer Corps, as well as handing me a scroll to give the Corps leader PERSONALLY. When I did so, I found out that the village elder was actually one of Kagaya Ubuyashiki's closest childhood friends! And the scroll had PERFECTLY summarized the situation of my birth and what had just happened, I was actually inducted WITHOUT HAVING TO GO THROUGH THE FINAL SELECTION. I quickly climbed the ranks, hoping that with each Demon I slayed, I would eventually draw Muzan back out to hunt me down for Round Two, inadvertently becoming the first Death Breathing Hashira upon my 17th birthday.

Demon Form:
HEAVILY based on "Full Form" Devil Kazuya from Tekken 7, but without "extra eyes" and having OC's face instead of Kazuya's

Death Breathing style: 
Form: Sweeping Scythe (basic lateral slash)
2nd Form: Soul Harvest (impale through abdomen)
3rd Form: Ferryman's Toll (sever ALL hands)
4th Form: The Dark Hunt (uses OC's Blood Demon Art to summon hellhounds to track a target)
5th Form: 1000 Cuts (self explanatory)
6th Form: Naraka's Pit (similar to 1000 Cuts, but as a tornado around target while starting at "victim's" ankles)
7th Form: Peaceful Rest (as Water Breathing 11th Form: Dead Calm, has a 6 meter radius)
8th Form: Midnight Breeze (slashes weapon in a way that also sends a "shockwave" capable of cutting objects, range up to 5 meters)
9th Form: Shinigami's Wrath (splits Blood Reaper into twin sword, then spins rapidly with edges out, can be stationary or mobile [become living Beyblade])
10th Form: Final Oblivion (vertical bisect)

Blood Demon Art: 
1; Razor Slash (turns slung blood into blades that cut ANYTHING)
2; Acid Bath (turns blood acidic except to self and humans, negating Demon regeneration)
3; Divine Inferno (similar to Acid Bath, but IGNITES the blood into WHITE fire instead of acid)
4; Insect Walk (allows self or those with special tags to "stick" to flat surfaces like an insect indefinitely)

Weapons: 
Blood Reaper (Wolf themed version of Dante's Inferno Death's scythe, actually manifested from Demon Form, able to also "shift" into a naginata and sword at OC's will, can also be temporarily "split" into twin weapons)
Onyx and Pearl (twin nichirin kunai that are blood enchanted to return to OC similar to a Jedi using the Force to bring things to them)

Miscellaneous:
Has the combined sensory ranges of humans and demons, along with the ability to actually smell pheromones and distinguish living beings by scent like canines
Can manifest and control Amaterasu at will, but also heavily tied to emotional state

Riku Kyskei, Death Breathing Hashira (Art by https://www.reddit.com/u/Squishyliltaetae07/s/tFK0aYmLGE)