Just tell me who she or it is already.

a girl that has a crush on Ash, she is Serena's nemesis

Yep

yes

Now I need to know who Serena is, and also, what this Miette did that makes people rank he lower than criminals.

Thank you for that tidbit though.

Serena is ash's childhood friend

But Ash is from Kanto, isn't X and Y set is France or whatever they called that region?

Yep

So did she used to live in Kanto or something?

If you check Bulbapedia, it says she was sent to Kanto against her will to attend Prof. Oak's Summer Camp.

Neat. I never thought of Ash as having friends before he turned ten or whatever age he's been, I always assumed he had a sort of Luke Skywalker, raised away from Pokemon because of something terrible that happened to his father type thing.

But, again, what did Miette actually do?