The Maiden maintains the Nexus, keeping its candles lit and watching over the souls of the beings trapped within.
The Maiden is Black is said to be one of the most powerful demons in existence and has the ability to modify the player's abilities in exchange for souls. Her eyes are occluded by wax, although this seems to have little impact on her ability to perceive the world around her. The Maiden in Black once had the ability to suck out the soul of any creature, instantly killing them, but it seems she no longer can.
She maintains a direct communion with The Old One, and is able to lull it back to sleep should the player choose to do so. She is a prisoner within the Nexus (a nexial binding is on her ankle) and cannot die, the only way she will be freed is to lull the Old One back to sleep. That freedom comes at a price for the only way to keep the Old One asleep is for her to sleep along with it. She can be permanently killed by striking while she puts the Old One to sleep. The player may also choose to kill her and take her place at the Old One's side, becoming a demon of immense power at the cost of the destruction of Boletaria, and eventually the rest of the world.
Though the Monumental believes she follows his orders, the Maiden in Black appears to have somewhat split loyalties. If the Monumentals did not show King Allant how to awaken the Old One, the only other character with that knowledge would likely be the Maiden in Black. She is also the method of the player's corruption as she uses Demon magic to embolden the player's flesh with the souls of humans, a power that "contradicts the essence of the soul." Once the five Archdemons are defeated, she recognizes the howl of the Old One, which she calls the "voice of yore" (time long past), and brings the player down to meet the Old One. There she speaks to the Old One as if she has been in constant communication, telling it that she has done as commanded and brought a new Demon to serve as a replacement for the failure that is King Allant.