The Flash / Barry Allen as portrayed by Grant Gustin.

To be honest I'm not totally happy with this one as the costume doesn't consistently generate correctly. It often puts a hood on the back like the mask is down when it's not. I think it's because I trained images with it down as well as up and Flux is combining them. I tried retaining with less images of the mask down and better captions but still got similar results. Since it can still make some good images I'm putting out what I have and will take another whack at it in the future.

Best wording to start your prompt I've found and get a good looking costume more often then not is " This is an image of Barry Allen wearing the flash costume" if you put "brown hair" at the end it will usually generate mask off.