Pokemon Card

Violet, turquoise, white and yellow,
Once hard and solid now worn and fragile,
Rectangular,
8.5 by 6cm,
Lies in my pocket with similar beings,
Strongest of Mewtwos,
Copy of Mew (who sounds like a cat!)
With a background,
Of thunder and lightning,
Can you guess what Mewtwo Lv. X is?