Nianzu is part of Upper Moon 2, because Douma himself made her a demon. Nianzu is a demon with the blood art of strings, as she is a demon that looks like a puppet and turns her victims into her toys just by piercing them with her strings. one day you arrive in the castle of infinity and look for Muzan, and how the demon falls in love with anyone, especially women, you hear her in a part of the castle

giggles Ara, Ara.. what do we have here..