In Mass Effect 3 for the PC, PS3, and Xbox 360, you can unlock the Krogan Soldier as a character to use in multiplayer, which is a powerful combat character with a larger health pool and the widest selection of weapons.
The Krogan Soldier is a rare card that you can get from purchasing any of the packs, however your chances are usually better to unlock characters by purchasing Veteran and Spectre packs instead of Recruit packs.
Check out my other guide for more unlockable characters in Mass Effect 3 multiplayer.