She uses the souls gathered by the player to level up by selecting "Seek Soul Power".
After all other major bosses are defeated, the Maiden in Black takes the player to The Old One, where he/she fights King Allant. After the player's victory, the Maiden in Black attempts to lull the Old One back to slumber. Killing her or sparing her impacts the ending of the game.
She does not become hostile when attacked and, while in the Nexus, resurrects shortly after being killed. Killing her does not grant souls, though the Needle of Eternal Agony's attack still earns souls when used on her.