Pandora was created by the gods 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.
Erika is a child that is seemingly full of innocence that is always cheerful. It is unknown if she really understands what is going on around her, however she does understand her father, Darius Ainsworth, to a certain extent.
One example was how Erika understood that Darius Ainsworth would disapprove of her bringing Illya to the Ainsworth's castle unless Illya was reduced into a powerless state which prompted Erika transfer Illya's soul into a stuffed animal. Although she originally wanted to brainwash Illya, she chose an alternative where she is still apparently pleased. Despite this, Erika seems to genuinely love her "older sister," Miyu, as she seemed to be really lonely as Miyu was gone in a parallel world.
Overall, she can be similar to Illya's Fate/stay night counterpart, as 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. Her personality drastically changes towards that of a cold person, closer to that of her brother, after she comes in contact with the mud of the Pandora's Box and recalling herself as Pandora.
When Erika reveals to herself as Pandora, her personality takes an even darker turn where she views everything with intense hatred and sorrow from the suffering she endured for thousands of years. This massive amount of negative emotions is mostly directed at humanity and the desire to finally die to relieve of her duty in opening Pandora's Box.
Pandora possesses the Noble Phantasm Pandora's Box where she is its key to opening it, but she is unable to open it because of the duty was given by the gods. Until she does, she is unable to be killed or permantly injured, making her immortal.
After living for so long and suffering so much trauma to her pysche, she lost some of the functions to act as a proper key in opening her box and now requires the power of two Holy Grails to do so.