I'm surprised by the amount of people saying the girl is retarded. She isn't. The author is (or at least their writing skill, I wouldn't want to insult the author). She didn't come here by herself, she was written like that by the mangaka. The one at fault here is Nakatake Shiryuu. I can't believe a real person would act like her, she's just too badly written, but so is the rest of the manga. So people, don't blame the character, blame the one who made her like that.
But I guess the editor is also to blame...