As we all know Yagyuu is very attached to Hibari but the reason for that is something I wasn't expecting. As I was playing through the first game again I came across some information which was interesting, this is something that isn't shown in the manga, at the end of chapter 12, when Yagyuu is in the process of rescuing Hibari, as she was about to be dealt a final blow she closes her eyes. This is a flashback at that time. Highlight below for spoilers. Yagyuu had a younger sister who suddenly, without warning disappeared right in front of her. She died in a traffic accident. As she doesn't ever want to feel the loneliness she felt after her death again, Hibari's existence became something special for her; a reason for her to live.