Rui's Dearest Lily (A KNY Fanfic)

Many years ago a pair of siblings was born just 3 years a part, the eldest had a weak physique and the younger was quiet the opposite. Despite their difference in physical compatibilities the siblings were very close. Until a man under the moon light came up to the older sibling, offering him a way to 'cure' him. Kimetsu no Yaiba belongs to Koyoharu Gotouge, not me