My prediction is that she is the reviving demon king. The demon king was defeated and sent to earth, either escaping or by accident. A magical girl happened to be there and engages in combat with the Demon king, the battle ends the demon king badly injured, but the magical girl dies. In an effort to survive it takes the magical girl's form/body and basically assimilates the magical girl in the memory. The demon king then forgets what it is originally, and fully leans into being a magical girl and the story will be either Haruna learning how to be a real hero, or her slow descent back into being the demon lord.