She thought she was just an ordinary little girl until the powers of a goddess rose within her. And with that power came responsibility. The responsibility for fighting against the evil ones who strove to destroy the world. The responsibility of protecting those who lived within the world. The responsibility of using her power, and not using her power when to use it would be an abuse of what she's been entrusted with.
Because those with power always had responsibility and this sort of power was no different. It wasn't always clear, what she fought for or who she fought. But it was her responsibility to understand as best she could and make all the right choices – because the power of a goddess was the power to change the world, and she wasn't just an ordinary little girl. She was a goddess. A young, fledgling goddess – but the responsibility didn't grow with age: she only understood it better, because the power was always there.