Please mind the criticism, but I personally reckon that the background colour could be changed to better cater for Miss Pauling's role and character, as it's basically a copy of the Femscout portrait which I don't find very fitting for Miss P due to the two characters having different roles and personalities.
Something that's darker, greyer and slightly bluer (kinda like that of the Medic or Spy portraits) might be more suitable for this image.
I should also note the 'blockiness' of the curl on the left-front side of Miss P's hair. Besides any of that, these images are absolutely marvellous and match that of Valve's style very well.
Wow, this is truly stunning. Your work is so close to Valve's official portraits, they should really hire you. May I promo you on tumblr? I'll be sure to credit you and link back so you can see the feedback.