Agatha is one of the three Ancient Ones who gave The Revolutionaries their magic. That makes her, indirectly, responsible for creating Silvast. She takes the form of a female when interacting with others, and bears a strong resemblance to Aurea. She serves Aldo, who is much more powerful than Agatha.
Agatha is the weakest of the Ancient Ones, since she locked a part of her magic inside Amnesty in hopes of controlling her. Amnesty was stronger than Agatha expected and still has control over her own body.