Eridna

Eridna is an Artificial Spirit created by the Witch of Greed, Echidna, who is currently in service of Anastasia Hoshin. First introduced in Arc 5, she later plays a pivotal role in Arc 6.

Similarly to her creator, Eridna speaks, sounds, and acts just like the Witch herself, however she has no memories of her past aside from her name. Because of this, Subaru nicknamed her Eridna (Scarfidna). She's also shown to deeply care about Anastasia, going to extreme lengths to make sure no one would find out about their condition.