Her name is Nianzu she is the Lower Moon 4 demon. Nianzu Is loyal to Muzan and thinks he is her bestfriend. Nianzu Is always smiling and acts very friendly but she can be a sociopath who showed or felt no emotions if she wants to. Nianzu Can regenerate. Nianzu’s blood demon art is called Puppet Transmutation. Her blood demon art allows her to summon threads from her fingertips and she can implant the thread in her opponents arms and legs giving her full control of her opponent’s body limbs.