Does not wearing a hijab invalidate a girl's fast?

The fast of an unveiled girl is valid, as the hijab does not affect the validity of the fast. However, she must fast and wear the hijab, as both are obligatory upon her from Allah Almighty.

The hijab is an act of drawing closer to Allah through what He has most belovedly obligated. Furthermore, not adhering to it may be a cause for entering the Hellfire, based on the Prophet's (PBUH) saying: "Two types of people of Hellfire I have not seen: ...and women who are clothed yet naked, inclining [to evil] and causing [others] to incline. Their heads are like the humps of leaning camels. They will not enter Paradise nor smell its fragrance..." It may also lead to being deprived of Allah's mercy.