The Destiny

The tale of Ymir is a myth that not many know about. At least inside of Paradis. The myth states that Ymir made a deal with the devil to receive her powers, that there are 9 titans, that no one can survive the curse and that Ymir is dead.

But...

Is that actually true?

Her brother always refused to let her join the scouts, even if the commander himself tells him that it would be one big step closer to saving humanity and obtain freedom.

Who is "she"?

Why does "she" play such a big part in humanity's destiny?

And why is she so desperate to join the survey corps? Why is she ready to sacrifice her life in a reckless act to join?

And what is this act?