While Call of Duty: Modern Warfare doesn't allow you to create custom avatars, it is giving us a collection of operators to choose from. These operators are separated into factions — the Coalition, and the Allegiance — and there is a really good diversity of people to choose from.
You can buy the operator edition of Modern Warfare that will give you some great new outfits for your operators and this is likely one of the microtransactions that will be available later on. It makes sense for you to be able to buy new outfits, and even new operators as time goes on.
Here is the current full list:
Allegiance
The Allegiance are the #BadGuys of the multiplayer section of Call of Duty. They are comprised of the Russian military, terrorists, and mercenaries, and they look almost exactly like the #GoodGuys. Given that operators don't do anything different, and what "side" you are on doesn't change anything, it's ...
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