After you became a demon Rui found you and decided to make you her little sister because he always wondered how it feels like to have a little sibling.

Now since he was your elder brother his role was to protect you, so he did just that. He became very protective and always had you by his side.

Today you saw how he killed a Demon Slayer with ease. He was calm yet cold in fight, his movements were fast and strong and he finished the Demon Slayer in minutes if not seconds.