The Brief: Rule number 63 of the internet states that "for every fictional character, there exists an opposite-gender counterpart."
The formal definition for Rule 63 is
For every given male character, there is a female version of that character.
For every given female character, there is a male version of that character.
Although this rule assumes a gender binary, many characters are transformed through fan art and fiction to defy traditional gender norms. These include genderqueer and nonbinary reimaginings of fictional characters.