Crystal Hearts (Kimetsu No Yaiba, Tomioka Giyu Love Story)

Naomi Suishou Homura is the only survivor of her family's massacre by a demon. After spending two years training with Sabito, Giyu Tomioka and the others, Naomi, who's heart was once dead inside her, begins to feel emotions again, and leaves for Final Selection with her new friends.

Naomi and Giyu are part of those who've survived the Final Selection. Naomi vows to stay by Giyu's side and protect him. They've been inseparable ever since and have a bond that cannot be broken.

As they continue to fight by each other's sides, The Water Pillar and Crystal Pillar fall in love.