First of all, great job! I really like the scene. I hardly see pictures in Summoner's Rift and I think you did a great job. Kat looks really awesome and I love all the details in her outfit. Overall, it looks very good.
As for a critique:
Her left hand (right side of the picture) seems like it needs to be bigger. Since it appears that her arm is sticking out a bit, objects and body parts closer to the "camera" appear to be bigger. Also, her head needs to be a bit lower on her neck. As for her neck, there shouldn't be a 90 degree angle on the right side of the picture. You can see a bit of the scalene muscles there, no matter how your head is tilted. I might not be a 110 lb. vixen like Katarina, but I'm doing her head tilt and I can see those muscles there, no matter what. Since she is so thin and has little body fat, those muscles will be even more visible!
While it's a stylistic thing, I'd probably smooth out that dramatic curve from her waist in relation to her hips. But that's just me and my style because I don't like drawing very thin people. Like I said, it's a style thing, but some people frown upon it. It's whatever you like and it looks like it fits in with the style, so I wouldn't worry about it.
As for her feet, I would suggest making the boot on the ground bend a bit more to make it look like she is kind of tiptoed.
Other than that, great job! I like your style! Every picture has mistakes, so I wouldn't worry.