#11 is the only photo that has a background that does not totally distract from the cosplayers...it's a fairly simple background, which makes HER the clear and obvious main focus of the shot. Some of the other background areas, like for example the really bright, sunny background in shots # 1,6,and 8, are just very distracting. In the half-body shot, photo #3, the background is a bit less in-focus because you used a longer lens focal length. Overall, the probklem I see is that the majority of these were shot with a small-sensor camera, and a short focal length lens, outdoors, with a small aperture...soooo...there is a ton of unwanted depth of field. That dreaded wide-angle look...just NOT good.
The whole idea is to make photographs that feature the cosplayer...and to minimize distracting elements like trees growing out of heads, and BRIGHT background color blocks, stairways, walking paths, and so on. The problem is how you set up. Needed fill flash on-camera? So that meant you needed a wide-angle lens from close-up. Better solution would be to have the flash set up on a light stand, and then move wayyyyyy back, and shoot with wireless flash, using a narrow-angle telephoto lens from 40 feet away.
You've just simply got to look through the viewfinder and get a clean background when you do shots like this. #8 is one of the best costumes of all, but paired with one of the worst backgrounds. At the very least, shoot with a longer focal length lens to minimize the angle of view behind the subject.