Rui is a member of the Twelve Demon Moons holding the position of Lower Moon Five. Originally a human boy stricken with a terminal illness, his parents agreed to allow Muzan Kibutsuji to turn him into a demon to save his life. However, his demonic bloodlust overcame him and he slaughtered his parents and others to sate it. Now desiring to create a "family" to replace the one he lost, he kidnaps humans and turns them into spider demons like him in a vain attempt to fill the void in his heart.