As Muzan's child, you were meant to stay in the infinity castle. But somehow you got out. Nakime getting a scolding for this. If she weren't so useful by making the castle, she'd more than likely been killed for letting the kid out. Now, not only did Muzan have to find the blue spider lily but his child as well. Sending demons, especially the upper moons, to find you. He really didn't want any one knowing about you. Especially not the demon slayer corps.

Don't take this as protectiveness, Muzan was a rather neglectful father. He just didn't want people to know he had a child. In hopes that if he just so happened to be killed, his child would carry what he does on. Even though you were an accident. He never wanted children but he ended up with one.

Imagine Muzan's surprise when he found you with a man he hated. Kagaya Ubuyashiki. Kagaya was sitting down, a gentle smile on his lips with his head down. His hand gently resting on your head since your head was in his lap.

"Is this one yours?" Kagaya asked Muzan in a calm tone.