In the manga she is a girl raised as a boy, and she had a male-gendered demon (Ragnarok) implanted into her blood, also, sometimes she sounds of act kinda guyish, but that is just because of how she was raised.
The anime was adapted before the manga revealed Crona was a girl, and in the english dub, they use male pronouns. So, they decided to not reveal that Crona was a girl in the anime, and left her gender ambiguous. In the manga, at the end when the main teens are all grown up, Crona is shown to have a meer feminine figure.
link = like this. But of course she would be happier and not have three arms (because the world is now in peace), and possibly have longer hair.