Mystery Woman, also known as Vega, governs Libra City from behind the scenes as the controller of the shadow organization, the Unseen Council. Her exact origins are undisclosed, but she possesses a direct connection to the alchemist who created the YOUKAI-NINGEN. She specifically resembles the original incarnation of Bela and shares a backstory tied to the alchemist's wife, positioning her as a figure with deep historical ties to the humanoid monsters' creation and existence.

Vega orchestrates a conspiracy targeting Bem, Bela, and Belo for capture, pursuing undisclosed purposes related to her control over Libra City. Her operations span both Upper Town and Outside, leveraging the city's structural divisions and societal discrimination to further her goals. While her motivations are not detailed, her actions indicate a sustained interest in manipulating or utilizing the YOUKAI-NINGEN for her agenda.

She maintains an enigmatic presence, operating through intermediaries and the Council's influence rather than direct confrontation. Vega serves as the primary antagonist driving the central conflict involving the trio across their struggles within the city.