rui was reborn as a demon and was very unstable but he was usually calm and he was later adopted by the demon king/muzan kibutsuji for killing and torturing his whole family. He also had a lot of blood demon art. Rui is also very lazy sometimes when he battles weak hashiras or humans. And he has a very good kick strength. He holds the rank of lower moon 5 and also became muzan's favorite along with some others.