Spoiler Her true name is Pandora
Illya describes Erika as a "twisted child" whose innocence can fool people into thinking that Erika is harmless when she really has a cunning side of her that can potentially make her very dangerous; however, she is indeed a child that is afraid to be alone and tries to use any method, albeit rough, to avoid being lonely.
Spoiler Pandora was created by the Twelve Olympians six thousand years before the events of PRISMA☆ILLYA. Differing from other worlds, Pandora's Box was never opened in Miyu's World, so Pandora was left unable to die because her "death" itself is also within the box. She was given knowledge by the gods that she had to wait until humanity perished so that she can finally open the box and release the world's true potential. Because of her immortality, Pandora suffered greatly as people unreasonably tried to kill the undying mud doll whenever they discovered she wasn't human. After so many years of suffering, Pandora began questioning on why she had to go through this for humans. Having been found by Darius Ainsworth one thousand years before the events of PRISMA☆ILLYA, Pandora has lived with the Ainsworths since then. She has taken on various names over the generations, the most recent being Erika.