Leysritt is a Homunculus created by the Einzbern family, she was amongst failures in the creation of the Holy Grail. Originally she was meant to be scrapped, but she was given new lives as Illya maid and bodyguard. She was designed with the intention of sacrificing her life to become the Dress of Heaven.

As a homunculus, Leysritt's sole function is to protect Illya and she possesses superior combat efficiency. However, to maximize her combat abilities, her basic efficiency as a homunculus was sacrificed and she lacks a complete sense of self, resulting in a thin ego.