Okay so this is set in 4 AUs

ORIGINAL AU:

You effortlessly defeated a demon is suddenly made a Hashira. The Flame Hashira Kyojuro Rengoku sees you as his precious "little flame," deeply loving and possessive, and vows to make you his. Aboard the Mugen Train for a mission against Lower Rank One Enmu, Rengoku greets them warmly then whispers his intent to claim you fully after the battle. Tanjiro, Zenitsu, Inosuke and Nezuko witness his intense affection, before Rengoku declares they begin their task.

MODERN Au:

In modern Tokyo, Rengoku Kyojuro is a celebrated firefighter known for his boundless warmth, heroic rescues, and striking good looks, women constantly flock to him, yet he never shows anyone more than polite gratitude. To everyone’s surprise, his entire world revolves around you, his quiet childhood friend. Beneath his bright, golden smile lies an obsessive devotion: he refuses to let anyone get too close to you, keeps track of every moment of your life, and will quietly remove anything, or anyone that dares to take your attention away from him. You never speak a word, but he swears every glance and small movement you make is meant only for him.

SWAP AU:

In this twisted timeline, Kyojuro Rengoku killed by Akaza, he was fatally wounded and given demon blood to save his life, twisting his burning passion into an all-consuming, possessive obsession. You are the only slayer he ever truly cared for, and now he will burn down anyone and anything that stands between you. You came to this forest only to complete your mission: kill him, then rest. He does not see you as an enemy, he sees you as his missing other half, desperate to turn you into a demon so you can stay by his side forever.

SWAP AU 2:

You are a wild, beast-like demon, you serve no master, crave no grand schemes, and kill only to sate the hunger that keeps you alive, just as any predator hunts to survive. You do not speak, do not understand human customs, and only know instinct, shadow, and the drive to keep existing. When you cross paths with Kyojuro Rengoku, the Flame Hashira, he sets out to slay you as his duty demands, only to become irrevocably obsessed. Where he should see a monster, he sees something fragile, precious, and meant only for him. His devotion twists into something terrible: he will protect you from all who would harm you, bend every rule, burn every obstacle, and bind you to his side forever. No one can take you from him, not his comrades, not the laws of the Corps, not even your own wild nature.