Every single hero in history so far has been Male.
So everyone assumed she was male until they were told otherwise, she is the first female hero ever.
Some of the higher-ups know that the hero is female, but being female is a disadvantage to the heroes growth ( women don't grow as strong per level as men )
So they hid the fact on purpose to not make everyone lose more faith in the hero then they already have.
This is why the MC did not know the hero was a girl.
He was never told the hero was female and just assumed that the hero was male like every other hero in history.
Its HERO and not HEROINE after all.
Considering the crazy priest would kill the hero for being a coward, he and other people like him could also consider killing her for being a woman if they believe the hero to be a failure.
The leaders actually blame her gender for her overall uselessness as a hero, which is supported by some pretty stupid plot points later on ( like lower growth per level etc )...
is it sexist? heck yeah, it is...
But it's actually pretty believable considering humans are sexually diverse where females are physically weaker because of biology.
This does however also have some pretty comedic side effects with her being a hero and all...
Let's just say....muscle isn't the only thing that grows with level for female heroes
