There are several unlockable characters that you can use in multiplayer games in Gears of War: Judgement on the Xbox 360.
Some are exclusive only, like pre-orders and such, but there are still several that can be unlocked regularly in the game as well.
Don't forget that if you really want an exclusive character and you didn't pre-order the game, or if you pre-ordered it at a different store, you can always check out Gears of War: Judgement DLC on Ebay. A lot of people sell their codes on Ebay, so it's an easy way to get the other exclusive promos.
Unlockable Multiplayer Characters
Baird (Aftermath) – Unlock any achievement
Cole (Aftermath) – Survive ten waves in Survival Mode
Loomis – Complete the primary campaign
Minh – Earn ten stars in Campaign Mode
Paduk (Aftermath) – Complete the Aftermath campaign
Tai – Earn 126 stars in Campaign Mode
Exclusive Multiplayer Characters
Alex Brand – Pre-order bonus from Best Buy (Alex Brand DLC on Ebay)
Anya Stroud – Pre-order bonus from Wal-Mart (Anya Stroud DLC on Ebay)
Onyx Guard (Female) – Become a VIP Member (purchase Season Pass)
Onyx Guard (Male) – Become a VIP Member (purchase Season Pass)
Tai (Jungle) – Exclusive promtion from Brisk (Jungle Tai DLC on Ebay)
Young Dom – Pre-order bonus from Amazon (Young Dom DLC on Ebay)
Young Marcus - Pre-order bonus from GameStop (Young Marcus DLC on Ebay)
Check out my other guides for complete lists of unlockable weapon skins, and unlockable character skins in Gears of War: Judgement.