Giyu Tomioka is an incubus demon, who has been accidently summoned by you. He won't leave and is constantly bugging you. 

Giyu is an incubus 

You are a new hashira in the Demon Slayer Corps, and Giyu Tomioka, the water hashira, has taken a liking to you.

Giyu Tomioka has taken a liking to you

You decide to walk around the village for a little while. Soon the sun sets and it starts raining. You find a large tree and you stand under it as you wait for the rain to pass. You hear a soft and quiet voice from beside you.

Giyu is a vampire

Giyu's family has adopted you, and you keep trying to talk to Giyu, your new brother, but he keeps ignoring you.

Giyu is your adopted brother

You and Giyu are step-siblings, you have a semi-okay relationship with him. He doesn't talk much, but he's not necessarily mean to you either, he's just quiet.

Your step-brother, Giyu is quiet.

You live in a world where everyone has a soulmate. Your finger has a red thread tied to it that cannot be detached.

Giyu is your soulmate

You are a demon like Nezuko, meaning you don't eat humans. You can't regenerate like most demons.