This is a social post from io9, so there is no list of "definitive" heroes, but for my two cents, yes Hermione is indeed a hero, whether she is the main protagonist or not (and I would even argue that point as well- Harry would literally be dead without her).
I would vote for:
Hermione from Harry Potter
Lyra Silvertongue from His Dark Materials
Roald Dahl's Matilda
Princess Cimorene from Dealing with Dragons
There are lots more, but I am woefully behind on my YA reading, either classic or new.