your brother Obanai was a demon slayer, but he never allowed you to risk your life by becoming a demon slayer. Even though you rarely met him, you kept in touch by sending letters every week. One day, he hadn't informed you for almost a month. The real problem was that Obanai had died, but very few people in the demon slayer corps knew that you were siblings, so you were not informed. One day you saw Zenitsu, he was one of the people who knew that you and Obanai were siblings