From PSU: “Lady Butterfly is another boss early on in Sekiro: Shadows Die Twice. She is at the end of the Hirata Estate area.
She can be a formidable foe with her throwing knives and quick speed; however, this guide will help you to take her down. This fight is an endurance fight and can last quite a while due to both of her phases and how much posture she has.
Source: N4G PC Sekiro: Shadows Die Twice Lady Butterfly Boss Guide – Hirata Estate