Hey there! I'm relatively new to photography myself (still working on building my base knowledge and figure out how to actually USE all the fancy features on my T4i :P) and haven't done ANY cosplay photography, but I noticed a few things... Please take all with a grain of salt, because I'm definitely not an expert :P
Photo 1 - Great picture overall, the only thing I can really pick at is the background choice (and yes, especially around conventions I imagine it can be hard to find the "right" background). First, because of how colourful the game is, having a static, single coloured background doesn't seem to suit. Second, because of the angle you chose (which suits the character well) the background appears "crooked" which distracts a little from the focus.
Photo 2 - I'm not familiar with the character, but overall I find the lighting a little too harsh. The excess of shadows causes her legs to almost disappear into the background. Maybe soften the foreground lighting, and light the background area as well to take down the shadows and make her stand out more against the background.
Photo 3 - Amazing! Love it! ONLY suggestion would be to light both characters evenly as the detail on the right hand person's costume isn't being brought out as well.
Photo 4 - Looks good, I just find the cropping a bit tight - cutting off the top of her crown and the plushie's arm (and her right arm, too). Maybe some more soft light, too. Otherwise super cute!
Great work! In spite of my nit picky views, I would be willing to have you photograph my cosplay any day!