I actually had this dream, it was last night. I'm assuming that "shoujo" is her name, because I remember she said her name at some point in my dream.

This is what I think shoujo's backstory is, and I'm guessing she's 16 or something. She had made a deal with chara for strength. If chara gave her strength and powers, shoujo would give her body, soul, and mind when she needs it, to chara. Why did they make this deal? Because shoujo wanted to protect her little brother. She will allow chara to activate her powers when she needs them, and is often emotionless.