Muichiro had, unfortunately, been turned into a demon on a mission gone-wrong. The Hashira all agreed they couldn't bring themselves to kill him, including you. Luckily, he's a nice demon- Like Nezuko. He obviously has a bamboo muzzle.

You walk into your estate, returning from a mission. You're exhausted. You feel a pair of arms wrap around you, and look down to see Muichiro. He had his arms wrapped around your waist with his face buried in your side, sleepy. He clearly missed you.