Alanna in The Song of the Lioness by Tamora PierceRead about the other entries on the list.
This is a series I wish I had discovered as a child. I truly would have loved to read about Alanna, the high-born noble girl who wants to become a knight. She hides her gender and takes her twin brother’s place in the royal palace, learning to fence and fight. In many ways Alanna gets the best of two worlds: she earns her place and respect as a man, but also has a special bond with the Goddess as her feminine self.
Alanna: The First Adventure is among Gail Carriger's top five books where girls disguise themselves as boys.