Looks like it's supposed to be a one-piece, but belted and with some of the top of the jacket bulging/hanging over the top of the belt they way material tends to do. The zip doesn't go all the way down to the belt either, so that might also contribute to the fabric riding up and out.
Have you ever seen Cosrevo's costumes before? Very expensive, and so far it seems one can only purchase from Japan, but their 2nd gen Allen Walker costume (http://www.cosrevo.com/shop/index.php?main_page=product_info&products_id=48) is extremely accurately done. Perhaps looking the pictures will give you some ideas?
The only thing they didn't bother making were the boots, which they re-use for other costume photoshoots.
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Have you ever seen Cosrevo's costumes before? Very expensive, and so far it seems one can only purchase from Japan, but their 2nd gen Allen Walker costume (http://www.cosrevo.com/shop/index.php?main_page=product_info&products_id=48) is extremely accurately done. Perhaps looking the pictures will give you some ideas?
The only thing they didn't bother making were the boots, which they re-use for other costume photoshoots.