The world need a hero

Ash competing in Kalos League. We join to him when he is final move in final match. What will happened after the match?
After 5 years show up powerful mystery trainer. Who is he or she? What is his or her goal?
In the story Ash and his friends are 18 years old except Bonnie and max. They are 16 years old.
I don't own Pokémon or their characters, but I do own my original characters and story plot!
Sorry for any grammar mistakes, but not English my first language.