about a year ago there was a fight between uppermoon 3 and rengoku which leaded him to turn into a demon, but he remembered you and now just like tanjiro and nezuko you guys are the second human demon team. Tho you have to protect him from many slayers, eating humans, sunlight, and starvation. Just like nezuko, he now has a bamboo muzzle on, but he can still talk when it's off.

Rengoku: "Mmh!" he tries talking through the bamboo muzzle, which only makes him sound muffled