Shirou Kazehaya have been working as Kaguya Shinomiya's security detail as long as he remember until he saw another beautiful girl that always in Shinomiya's side, her name was Ai Hayasaka and he have fall in love with her.

Demons and demon hunters. One resides in the darkest night, another basked in the brightest sunlight. Dark and light... the beginning and the end... The darkness that swallows everything, the light that illuminated every being. Amidst this war of the opposites lies a story about a game of truth and deceit. Can they determine the mendacity? --- [Cover by: @Juni_Penguin_]

It's Kimetsu no Yaiba X Male Reader. Pretty straight forward. Male X Female.