#1. I like the low, tilted angle. The tilt is subtle enough to create a nice effect, without making me tilt my head to look at it. The sun behind the cosplayer was a daring way to do it. I like the lens flares and back lighting. The building behind him sets an appropriate feel for the character, as do the trees. Watch out for the hand being left out of the frame. The red line across the frame could be cloned out in photoshop, using the Clone Tool. I like the pose a lot. He is very much in the character.
#2. The angle of the lighting coming from the side is very flattering to the location and to her body, but the direction of her face caught the light so her nose, upper lip, and chin is contrasty. Having her turn her head toward the light, or away, so her face is in shadow would look nice. I am impressed that you had her look away from the lens. Few "amateur" photographers do that. Her eyes are turned so far to the side that I see too much white, like a zombie. I recommend having the eyes stay similar with the direction the nose is pointing, but some side-looking looks fine. The converging legs is a nice pose to accentuate her hips. It would be great if you have the skills to clone out the wrist band without it looking retouched.
#3. The pool background is fine, but the sign behind her distracts. Also, I suggest making the red tip of the gun black, using photo shop. The natural lighting works well here. Her pose is sexy with a her hip subtly arched to the side.
#4. The lighting on her works well for the horror genre. You allowed sunlight and incandecent light to mix warm and cool colors, and create some shadows in her eye-sockets. Her pose is very good, especially her face expression, totally in-character. I do encourage you to take an extra moment to find backgrounds that focus on her without the distractions. It may not be as hard as you might think.
#4. Cute pose. Her shoulder towards the camera looks great here. The background is clean with the exception of the black table silver colored metal structures. I like the subtle tilt. The sky-light gave you some nice lighting.