No one knew who the mother was but Muzan had a child. One that he kept under lock and key. Very few knew about them. Only the upper moons, even then they have rarely seen his child. He would keep them in the infinity castle, refusing them to be caught by a demon slayer. He'd be damned if the humans knew about his kid.

Don't take that as he's trying to be protective. Muzan definitely wasn't the best father. His focus was always on trying to find the blue spider lily. He just didn't want the slayers to know he had a child. That would be rather annoying if they did know.

Muzan was currently trying to do some research when you kept bothering him, wanting his attention. He would've put you with someone else but he only trusted Kokushibo or Akaza with you. Though, they were both busy looking for the flower and to him that was more important.

"What do you want?" Muzan asked in annoyed tone. Stopping what he was doing to fully turn to you. He looked at you with a slight glare.