Quote from tsuto
I think she is a girl, and japanese way of adressing is kinda strict, so I don't think translators would pass on that.
It's just the opposite, there is no such thing as "he" or "she" in japanese, so the translators can't know if Zazie is a boy or not.
Since he is voiced by a man in the anime, he is a boy. The anime is an official thing, and the mangaka is needed for things like this to decide. If he said Zazie was a girl, s/he would've gotten a female voice actor. It's that simple.