I am a Women's Studies graduate. The reasoning of feminist writers is simple: "women" are human beings who are gendered female. Non-human animals (e.g. sheep, cows, pigs) and some physical objects (e.g. ships, countries) can also be gendered female in the English language (e.g. "Titanic's maiden voyage began shortly after noon on 10 April 1912 when she left Southampton on the first leg of her journey to New York."--Wikipedia). For feminists, it is very important to treat women differently from animals and physical objects. Therefore, the term "women" is always preferred when referring to human females, rather than using the term "female," which could apply equally well to a sheep or a ship.
As far as I am aware, most feminist scholars would find it perfectly acceptable to refer to "female characters" in novels or plays. That is because a character is a literary device and not a living, breathing human being. However, if a feminist writer chooses to use the word "woman" in referring to characters in a novel or a play, the preference would likely be for the plural form ("women") for the same reason that it is now standard practice in the English language to refer to Americans and Germans in the plural, rather than writing treatises on "the American" and "the German" character. In the 21st century, we understand that there is no single typology that could stand in for an entire nation that has millions of citizens with different preferences, habits, and so on. Thus we speak of Americans rather than "the American". In the same way, there is no single typology of a woman or a woman character. Because there are different ways in which such characters can be portrayed, with different character traits, preferences, habits, and actions, it makes sense to feminists to use the plural form ("women")as an indicator that no one character can stand in for all that it means to be a woman.