The Shiny Zygarde

A certain little Bluish creature called a Zygarde core is now awakes from its slumber to find its long lost brother who had been reborn to save the world serval times, after his brother did that purpose the shiny Z2 is now looking for the shiny Z1 to be able to give back his memories, to show him his true self, to finally reunite and truly become Zygarde the shiny order pokemon.

But who could shiny Z1 be?
What does arceus have to do with this?
And why does Z2 want to go to kalos?

Find out as the journey continues