Photo #1 This would have been fine with a brighter exposure and a bit of post editing to brighten the cosplayer, and darkening the background a bit. Then add some magenta, and some red, with a hint of yellow to remove the greenish/cyan cast. You could make the lighting more flattering using bounce flash, or a flash on a light stand with an umbrella for soft, portrait lighting. That could create some nice modeling on her face (flattering highlights and shadows to create a more 3D effect).
Fill flash could be a solution, but that can imply using a flash on the camera pointing directly at the subject, which generally gives unnatural, amateur, or standard photographer results. However, if you bounce the flash off neutral walls or ceilings, or a reflector, you can get very professional, portrait studio-like lighting. That would make your flash more of a key-light, rather than a fill-light. This technique can give you a more natural looking effect, rather than flattening the lighting with direct flash.
Photo #2 This photo has a wacky laugh going on. I am not sure if she is in-character or if it was intended as a in-the-moment shot of a cosplayer having fun. If it was the latter, then mission accomplished. There is a lot of space above her head, only to show a distracting window rather than show more of her costume. Framing her to the right of frame would make a nice composition to lead the action of where she is looking, rather than putting her in the middle, as you did. Adding some red in post editing would rid her of cyan skin tones. Adding a bit of brightness by boosting the highlights and a bit of the mid-tones, would also make the image more snappier.
Photo #3 The available light you caught here is very flattering. Your color balance is very nice here, and your exposure is very close to ideal. Adding a bit of boost to your highlights would bring out the whites in her eyes, and brighten her teeth a bit too. I would use the Sponge Tool in photoshop to desaturate the yellow teeth. There is a bright distracting object protruding near her head. Since you missed that during the shoot, the you can photoshop that out, if you are good with the Clone Tool. I like how you have her angled with her shoulder turned toward the camera. That is usually a flattering angle, as it shortens the width of her shoulders. The green plants behind her are a nice background. Bringing her farther away from the background can give you the option of making it more out of focus, but what you have is still nice.
Photo #4 The cosplayer's pose looks good, but I would like to see her eyes, unless that is part of her character. The composition is close to good, but cropping the feet takes away from it. Also, her hand is a bit close to the edge of the frame. Composing her a little to the right would give the shot more balance. Your exposure was too dark, but is fixable in photoshop. It is possible to brighten the cosplayer while maintaining the pretty water fountain's colors and brightness. Watch out for the water looking like it is shooting from the top of her head.
Photo #5 A brighter exposure again, would be a good thing. Your composition has a nice balance here, but a lower angle would have made her legs more long, and her head a bit smaller, which is generally a good thing. It not only tends to make the proportions more flattering, it also make them look more powerful. If you backed away a bit, you could use a more telephoto lens, which could make the entire background a wall of water, cropping out the hotel structures around it.