You have to keep in mind that French is a much more 'gendered' language than English, so we tend to use the feminine version of the word as long as there is one. Using 'hero' when the sex of the person is known will definitely be perceived as incorrect, although actually right if the gender is unknown.
But there are some French word who aren't gendered, called épicène. Some example include :
- un/une photographe (a photographer)
- un/une gendarme (a policeman)
- un/une adulte (an adult)
- un/une partenaire (a partner)
Please note that those words have no feminine alternatives (since they are non-gendered), so I'm not sure that's what you asked for.
Strange thing, there are words with no feminine counterpart. For example chauffeur (driver) has none. To name a woman driving a vehicle, you would have to use the term conductrice...