Hol' Up - He was a she the whole time!?
Yup. And it was even better in the light novel. Their version was a little more feminine in appearance; this Kiel doesn't really look like a girl in her early teens, whereas the image in the LN of this scene was definitely that of a young girl. Also there were a lot of funnier bits, like how after Filo gave an exceptionally thorough and eloquent explanation of the depth and meaning behind mating, Kiel's response was "there's no way I could do that with Shield Bro!". Yeah, the moment she learned about what her body's designed to do, she immediately thought of Naofumi.
Also, she was a bit more willing to wear a dress in the LN, and when she did she tended to get rather shy since she's not used to it, which only enhanced her charm. As a result, Naofumi would actually make a lot more anytime she helped him out on his sales.