Eustia Astraea is the main heroine of the visual novel 'Aiyoku no Eustia'. She is a mysterious young girl with angelic wings, found imprisoned and suffering from a disease known as the 'Feather Disease'. Eustia's presence becomes central to the story as she is closely tied to the fate of the city and its people. Her gentle and kind demeanor, coupled with her determination and hidden strength, make her a key figure as the protagonist and other characters seek to uncover the truth behind her existence and the city's secrets.