The 2nd guardian

Rey, a guy who died because of an unknown reason, was born as the brother of the guardian who will fight the invaders from another planet. What will he do? Will he help her sister? Will he run? Or will he do nothing at all? Watch as he travels through a weird post-apocalyptic world.
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First-time writing, don't know what to do, I'll just put here what I had in mind because it has been bothering me since I have played the game. Also, a lot of content in this book is headcanon so it may become inaccurate when the guardian tales story advanced.