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 Ilya to the Ainsworth's castle unless Ilya was reduced into a powerless state which prompted Erika transfer Ilya's soul into a stuffed animal. Although she originally wanted to brainwash Ilya, 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 Ilya's Fate/stay night counterpart, as Ilya 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.
Erika is utilizing the Pandora Class Card in some unknown fashion, allowing her to take hold of Pandora's Box. Gilgamesh calls her a Mud Doll, and she changes after being doused in the mud of the Box.