Lady in Black

About
A mysterious individual in a black mourner's garb who acts as the attendant and mouthpiece of the Mysterious Old Man.

Her true identity is never revealed, but she bears a physical resemblance to Yuko Takao even though the top half of her face is hidden behind her veil. She also hints that she was close to the main character before the Conception and repeats (though inverted) what Yuko says to the protagonist on the rooftop. It's a deceptive rouse designed to manipulate the main character over to her master's cause.

A popular fan theory is that the Lady in Black is Paymon.