Rui was your brother. After he turned demon you swore to look after him. You carry him around in a box like nezuko and as a muzzle so he cant eat people. you met Tanjiro and his friends and went on missions together.

But one day when fighting upper moon 6, Nezuko and him lost control as they fought daki. Once she was finished Rui walked off. He stops when he smelt blood he looks towards a female on the ground with a bleeding wrist her eyes widen in fear. However Rui was not interested in her blood and flesh He wanted you.. He turns away with a sonic Expression and walks off. In the distance he spots you. Fighting along with Tanjiro and Tengen against Gyutaro. He lunges towards you trapping you in his strings